If you're trying to keep the replacement cost low, I'd look for a used 8 or 9 speed RX100, RSX, 105, or Tiagara rear derailleur on eBay or on your local Craigslist. You should be able to find some for between $10-$20, including shipping (if necessary).

If you are patient, you should also be able to find a new 8-speed Sora rear derailleurs (and front derailleurs) on eBay (try looking for Seller Crossroads), if you wait and watch for $10, or less including shipping (I usually don't bid more than $3). I've gotten a couple fronts and rears for my parts bin in the past year (I've bought a number of used 105 8 & 9 speed derailleurs to put on bikes during the same period for $1-$15). IMHO Sora is good for spare parts bins, and emergency spares, and will work fine when new, but they aren't very high quality, and Shimano budget derailleurs often don't hold up very well over the long run. Sora and Claris have stamped steel parts that will rust where RX100, RSX, Tiagara, and 105 are alloy (lighter and more rigid).