Google 'fake followers' and visit the website ranked 1st. You can log in with any of your accounts and then type in the name of the Twitter account in question. It's not 100% accurate, but it's near enough. Look at the results it spits out and common sense takes care of the rest.
Look at the number of retweets it gets. Tweets with links in, naturally get fewer, but for 50,000 followers, you'd expect tweets to get about 60-70 RTs. If it's only getting like 10-11, I'd be suspicious. This doesn't mean you'll get that amount though, as it depends on the subject and also how many real followers they have, which you'll have a good idea of by now.
As for price. It varies. I'd be offering between $250-500 if it's a decent account, but only if it relates to what you want to tweet.
No point buying a twitter account on gardening, when you're wanting to tweet jokes.
I've got over 2 million followers now, but I've only ever bought 1 account and that was for the name - it had about 80 followers, but now has over 300,000. If it's a good username then that effects the value too.
Warning: When some people sell accounts they'll do a RT/Tweet for you but then RT it to their larger accounts to make it look like you're getting your monies worth. I had someone mail me asking for $300 for an account that had over 100,000 followers. I did the Fake Follower check and 77% were fake! Their follower count then started dropping and is now about 60,000 and still dropping. He did a RT for me, but noticed other accounts kept being mentioned in the replies, so knew it was those accounts that he'd used to RT as well to bump it up. So there's several things to look out for.
