While it's true that GPU prices are an important factor, it doesn't matter how great of a deal you get if you can't get the card to produce good results. What are the hash rates on your other 5830s? I've found that the 5830's are about the best cards that are presently available which provide the most "bang for your buck". For example, if you can press them above 300Mh/s, which is a reasonable rate for that card, you're already getting over 2Mhash per dollar spent on the 5830 card as long as you paid under $150 for it! A quick search on Newegg shows a Sapphire 5830 for $129.99, which is well within these parameters. Not a bad ROI at all!

The post before mine mentions a 5870 card for $189, which would be a good deal as well, however I can't seem to find a brand new 5870 listed for sale anywhere on the web, let alone for a price anywhere close to that. The Radeon 58xx cards seem to be harder and harder to come by anymore. It wouldn't surprise me to see the Sapphire 5830 from Newegg disappear soon as well. At $130, it's not a bad deal at all, but again it all depends on what hash rates you can get from your cards. Good luck!