OP, as long as you can afford it (in the MMM sense, and it sounds like you can), I say go for it!
However, I'd personally be uncomfortable inviting friends, especially since, as you say, they are not as responsible as you are. Sure, they make their own decisions and can always say no, but if you know they have little extra money, it seems, well, unsporting to dangle this opportunity in front of them.
Okay, that's out of the way! For flights, I'm signed up for travel deals newsletters on Travelzoo (the best in my opinion), Kayak, Airfare Watchdog, and Fly.com. Fly.com has a fare calendar for deals from most major US cities (Chicago is definitely on it) for the next 90 days, which is a good way to benchmark prices. Airfare Watchdog has something similar, but I think the timeframe is different. I've also had good luck with bidding on flights using Priceline's Name Your Own Price feature.
I often get notification of specials from Travelzoo about deals for CheapCarribbean.com. I have zero experience with the site and its packages, so this is
not an endorsement. But for instance, the site is running a $24/person/night special for some place in the Dominican Republic:
http://www.cheapcaribbean.com/landing-pages/1823.html