There's essentially 2 groups of expenses: startup and recurring. Obviously if you're buy a new boat, then there's the additional expenses for all the new equipment and gear (batteries, motor, PFDs, toolkit, first aid kid, etc. etc.) But those are 1-time expenses.
After that, it's just normal operating and maintenance expenses. You'll have a small engine, so gas and oil won't be that much. Neither will towing, licenses, and insurance. Keep in mind you'll need both a trailer and boat license, and possibly an engine license if your state requires it. It sounds expensive, but for a Jon boat, it shouldn't be very much at all. Throw in an annual tune-up, and that should be your only expenses after you're all set up.
Oh, and get an aluminum prop if your engine is less than 150hp. You don't really need a stainless prop for lower HP, and you'll save bucks.
Here's a great checklist to help you get started: http://www.bassresource.com/fishing/boating_checklist.html