They've got an 11-30 tiagra for 15.99 if your derailleur and chain can take it.
http://www.cyclechat.net/threads/tiagra-long-cage-v-short-cage.56905/
Have a read of the above thread it might help. You could look at the specs for your rear mech and it might tell you it won't work but if you buy it and try it on the bike it might. Set the chain length for big sprocket big chainring and the rear mech might not be able to take up all the slack if you shift to small sprocket small chainring but you wouldn't ride in those gears anyway!