A lot depends on how well the previous owner took care of the boat. He may well have not taken care of a lot of little things that any new owner will want fixed. The bigger and more complicated the boat, the more that may need attention. Does the new boat have upgraded electronics that you may want? If you bought my older used boat, you'd be in good shape and your expenses would be minimal. But, be aware that you buy a used boat from a private party, you'll have to pay sales tax (in Illinois that's 6.25%) plus a title transfer fee of $50 plus a registration fee to the DNR. In addition, you'll need to get license plates for the trailer -$18.
Batteries for the starting engine and trolling motor need to be replaced every 4-7 years at $80-$150 each. If you aren't able to do work on the engine and mechanical systems, you'll have to pay a mechanic maybe $100 an hour to fix the things you can't. Using the boat requires gas and 2 stroke oil plus fuel for the tow vehicle. So how often you fish, how far you drive to the water and how much you drive around while on the water will affect your costs. If your tow vehicle isn't set up for towing, be ready for increased maintenance cost for tires, transmissions, brakes and suspension which wear out more quickly when towing.
In the end, you may spend $400 or $500 a year if you are lucky or you could spend $2,000-$3,000 or even more. I have a friend who fishes a lot more than I do, pays others to do his repairs on a 4 year old boat and probably averages at least $300 a month in expenses.