Don't forgot to put some money aside for a decent helmet too. No matter what type of riding you do, protect your noggin.
Carrie, from what you've described, I'm thinkin' you want a cruiser bike. Something you can just use to stroll around town, so you can check out everybody or so they can check you out. =)
Check out this vid as it might be helpful.
http://www.youtube.com/watch?v=QQri9pZqtjM
I'd recommend checking out different saddles to find the right one for you. And if your ass bothers you that much when on a bike, you may want to consider investing in a suspension seatpost. Don't have much experience with those myself though.
As for price range, I'll agree with what everybody else has said so far. You get what you pay for. I'd recommend staying away from department store bikes as the build quality just isn't the same as one you'd get from a bike shop. I don't think I'll be too helpful with price as I've dropped over $800 on upgrades to my bike so far this year. I think I may have spent more on upgrades than I've actually spent on the bike itself. So if you're looking for something cheap, I'd recommend getting a quality used bike instead of a new department store bike. Kinda like saying I'd rather buy a quality used Toyota vs a new Chrysler.
If you ever consider doing some fast flowing singletrack riding on a cross country or all-mountain bike, let me know and I'll join ya. =)