How I plan on getting a bachelor's degree for ~15k.
1. Go to a community college in a county that you are a resident for your two-year associates. The money I'm getting from Pell grants is paying my 18-credit hours per semester tuition right now (Roughly $120 per credit hour tuition). Books on top of that are costing me about 300 a semester (buy cheap old editions as much as possible). I've gotten scholarship money to cover that, and I'll also get Tax credit money.
2. Transfer to an in-state 4-year public university. Tuition rates will double for me during this transition, but YMMV (I'm in Maryland FYI). The hike in tuition will cost me about 7k a year for two years. Hence ~15k out of pocket for a degree.
Moral of the story: If you really want a premier education where you will be learning from those on the forefront of research, by all means spend 100k at an elite private college. If you want to be part of academia or research projects this is probably the route you have to take.
If instead you know what career you are going into and you have a desire to learn those skills on your own, then go the cheap route and just get the piece of paper to prove to HR you are worthy of a job.
*Obviously some careers might not fit this model, but I'd guess most people throwing money away at private universities could go this route with the same basic outcome.