We've never paid for tweets (as far as I can remember), but we've had a lot of tweets from "celebs" and large Twitter accounts. I thought these tweets were the best thing since sliced bread initially, but I've now realised the benefit to my business is practically nil - save for a few sales, and some extra followers.
While most of the tweets resulted in large amounts of traffic, the sales were nothing to write home about. The tweets we receive tend to form part of larger collaborations, so they're not something we overtly ask for - but a plug is a plug, and I'm not going to complain. If I had to pay extra for the tweets, I wouldn't bother.
It's worth noting that the people who've tweeted about us are about as targeted as you'll get for the niche that we cater for. The fact those tweets have been a damp squib leads me to believe it's probably not worth bothering with. If you're Lord Sugar plugging your companies through your own Twitter account every day, then you'll probably do OK. If you're paying someone else to tweet about you once or twice, you may as well not bother. It's good for the ego, crap for the wallet.
I certainly wouldn't bother going down the route of paying for tweets, there are far better ways to blow an advertising budget.