Well, I’ve never owned a comic book shop, but I have a friend who worked in one for several years and is now fixing to buy his own, as soon as he can raise the money. I’ve also attended a discussion panel with him at the San Diego Comic-con regarding the nuts and bolts of starting a retail comic business.
What I can remember:
Rent is definitely your biggest expense.
The comic book market is foundering a bit at the moment (as opposed to, say, 15 years ago), so it’s difficult for even the successful stores to turn a huge profit.
There’s really only one comic book wholesaler from whom you’ll be ordering your inventory (Diamond), and they can be a bit expensive, furthor cutting into your profit margin. And they don’t take credit. 
Competition is fierce, and location is everything. In a moderately sized city such as you describe, you’ll probably be competing with several other shops. I think my friend has a good chance of being successful because he lives in a small town (~80,000 people) with nothing resembling a comic book shop within 50 miles. In order to get their comics, baseball cards, games, etc., people either order by mail or take an hour-long drive.
Most of your revenue (from comics) will come from new issues and trade paperbacks. Unless you’ve got the room, back issues are a waste of space.
Most successful comic book stores have needed to diversify in order to stay in business – they also sell sports cards, games, action figures, clothing, and other collectibles. Some are branching into the field of web-based mailorder, since it’s a relatively cheap thing to do, especially if you have a bit of computer knowledge.
As for startup costs, I can’t give you a concrete figure – you’re going to need to figure that out yourself – but as a general rule you shouldn’t go into the business unless you can afford to lose money for at least six months. In other words, six months worth or rent, inventory, and any other costs you can imagine. This can be 20K or 120K, depending on location, store size, etc.
As for income, I haven’t the slightest idea how much you could make, but I’m sure it all depends on how well your business does and how long you can stand to be in the red before you actually start making money.
That’s all I can think of at the moment. I apologize if I sound like a wet blanket, but any small business is a risky investment. A wise thing to do would be to talk to a few comic book retailers in your area and learn from their experiences.