Lack of teeth contrbutes to jaw/facial collapsing as you lose structure in your mouth -- it really is a last resort.
Dental schools are one good option for low-cost (or sometimes free) care!
Also, there are many dentists that specialize in working for low-income communities. They tend to offer payment plans or accept discount or insurance plans that others don't. Perhaps coming up with a treatment plan that handles one tooth every six months or so?
It is too bad that Medicaid covers dental in only a few states (and even then, it is sometimes only for children), especially since missing teeth contributes to unemployment (it's true; there are studies; but think about it, how many toothless cashiers and wait staff do you see? not many . . . people don't want the toothless to represent their business . . . it is a vicious cycle for the low-income folks who can't get dental care).
There are many programs that will have "free dental care day" annually in some community or other that you might be able to access (but again, it is often focused on kids). But check out your community and see what's available.
Have you looked into taking a small personal loan to cover the costs? Seriously, dental health is very important (not only due to the pain, but bad oral health can lead to infections and even heart problems).
Good luck!