Like pp's said, just offer what you feel comfortable doing. 80 guests at a country club seems a little excessive. And your MIL doesn't have to know about your financial issues-the best way is to say that since you're having a baby, you've cut back for baby related expenses.
If it were me, I would tell my MIL that I would like to help as much as I could, but there are so many unknowns, that I'd do my best to help with as much as I could, but we'd have to play it by ear.