Short version: Yes. And no. It's not about the ISK, it's about the risk.
Make sure you've read my two previous posts: Tippis' Rants - Faucets and Sinks, and Incursion vs Mission Income, along with Tippia's post Faucets and Sinks which spurred all of this blathering of mine.
So that's two posts in which I refuse to comment on whether Incursions pay "too much".
I have established that statistics on mission income are misleading. Just as an example, I make more than 5.7M ISK per mission, much less per day, as Tippia's analysis indicates. Per day my mission income is even better, when I'm running L4 missions. I have established some ballpark figures for income from Incursions or missions based on my own experience.
But is this income "too much"?
Many of the people I run Incursions with are people from null sec or low sec alliances who are padding out their wallets so they can engage in discretionary spending. The alliances will cover the costs of ships lost during alliance operations (not many capsuleers can afford to fund their own supply of dreadnoughts and titans, just as an example), but the alliances will not cover "voluntary" losses such as ratting ships, or ad hoc roaming gang fleets. These folk jump clone to hisec, run incursions until their wallets are nice and fat, then head back down to low sec or null sec where their alliances live.
People like Darius III feel that Incursions are drawing their alliance members away from the indentured servitude in which they belong. So Darius III and friends set about shutting down hisec incursions, by drawing more people away from null sec. Being null sec denizens, they got bored of that quickly.
I would suggest that hisec incursions are too rewarding, if it gets to the point that null sec alliances are losing active members to hisec incursions because hisec incursions are more fun than alliance structure bashing blob fleets. Or perhaps structure bashing is not rewarding enough? Yes, that's a joke: structure bashing in null sec sovereignty battles is even more boring than courier missions. I've been there, done that.
I didn't mention ISK there, since before Incursions came along I knew PvP-happy folks who would hang out in hisec for weeks, running missions in their "downtime". They'd head back out to low or null once their wallets were full.
So from my own experience, hisec incursions do not pay out "too much". People looking to make ISK will generally be looking to build up a fund: their aim is to get their walled up to X ISK. Once they're done building up that fund, they get back to the activity which soaks that ISK right back out of their wallet. That is, they get back to PvP in a PvP oriented game. The horror!
Why don't these people run Incursions in low or null? Incursions come with an iron-clad "zero hot drop" guarantee, so you're not going to be out in a fleet of 10 battleships and suddenly facing a swarm of Pandemic Legion super capitals trying to pad out their kill boards. Yet there are null sec residents — people familiar with the risks of nullsec and how to manage them — heading to high sec to run incursions.
It turns out that these folk come to hisec because they can actually get to all the available Incursions out here. There are, at times, three incursions in null sec. There are more null sec regions than hisec regions. There are, at times, three incursions in hisec. Thus the concentration of incursions in space you can safely navigate to is far higher in hisec than in null sec.
If you're a nullsec resident looking to boost your funds, and you don't want to pay for a PLEX to convert to ISK, what do you do? Wander across a few regions in the hope that you don't get caught in a gate camp, so you can run Incursions in someone else's back yard, or head to hisec where you can run Incursions with impunity?
Hisec incursions are simply too accessible. Have a look at what happened when Darius III and friends shut down the hisec incursions the moment the Mothership was found. More people started running low sec incursions. Players are looking for ways to inject X ISK into their wallet. They do what they need to in order to get that ISK. Some will buy PLEX, others will run missions, others will run Incursions.
It turns out, Incursions pay out enough to lure people into low sec and null sec Incursions. Does that mean they pay out "too much"? No. In my book, they pay out enough to draw people into low sec, which level 5 missions couldn't do.
Akita T was saying for a long time that low sec needs to pay more in order to get more people to head out to low sec in search of profit. Planetary Interaction is one of the things which pays enough in low sec that people will head out to lowsec in search of profit. Many of the people I know have bases in low sec, but entirely rely on planetary launches to retrieve their goods, rather than risk approaching a POCO.
When hisec incursions are not easily accessible, people head out to low sec to run incursions. Planetary Interaction and Incursions are paying enough to draw people out to low sec. Level 5 missions do not pay out enough to draw people to low sec (King Rothgar runs level 5 missions in a carrier: he's a lowsec resident who runs level 5 missions because they're in lowsec. He's not in low sec because he runs level 5 missions).
But hisec incursions are easily accessible — perhaps too easily accessible. There is a no-brainer decision to make: hunt an Incursion down in dangerous space, or travel to an easily accessible low-contest Incursion in hisec? There's an old saying in the drug dealing world, "first one's always free." When you're dealing in addictive substances, it is to your advantage to give potential clients a free fix. They'll get addicted, then they'll do anything to get the next fix. CCP needs to take a page out of that playbook, in my opinion: have just enough hisec Incursions that people can get a taste for it, but don't have enough hisec incursions that people can run incursions as their only in game activity. People who are prepared to take the risk of running Incursions in low sec should be entitled to the generous rewards.
This post does not consider the relative payouts of Scout, Vanguard, Assault and HQ sites within an incursion constellation. That's meat for another rant.
PS: "It's not about the ISK, it's about the risk"is catcher than "there are a number of factors including risk, availability, accessibility and ease of finding likeminded individuals." Please accept that I do exercise poetic licence from time to time ;)
 As an example: "CCP, does it feel good to waste a lot of your developer time", or "Introduce FULL auto-balancing rewards…", or "hisec and zero sec planet resource ratings".