I've been mowing part time since 1992, back then I used a small 5' mower. Every time I mowed a large tract something would break on the mower or the tractor. The equipment just was not heavy enough to take the beating of the terrain. Kept buying larger equipment until now I have a 90 HP tractor and 15' batwing. Just like several folks have already mentioned I read a lot on the forums about what other contractors are doing (especially FWJ) and have made several changes in my business practices such as general liability, CDL, commercial insurance. Mr TMC_31 had an excellent post on page 3 of this thread.
After my minimum is met, I charge $30+ an acre. I base my estimates on mowing 5 acres an hour. Unless there are a lot of trees I can hit that mark pretty close on most jobs. I don't give discounts unless I mow monthly for the same customer. My reasoning is that I could be doing another job making the money I'd lose money if I charge less. Being insurance poor is very expensive, so far my insurance company doesn't give me any discounts to pass on to my customers. So I would have to charge at least $3K for the job. Most of my jobs are 10 to 30 acres.
Would I do 100 acres with a 5' mower? No way. It's to hard on small equipment, not to mention the toll it does to the body. I couldn't hardly walk for a day after mowing 10 - 12 hours on a small tractor. And more importantly unless I have insurance the risk isn't worth it.
Good luck on your decision.