If you really want to go compact on the cheap, you could look at picking up an old MTB crankset. That's what I did with my old 6-speed bike. I used an old Deore crankset with 110mm/74mm BCD, took off the granny ring and removed the 5 3mm spacers that were between the granny and the crank. The Deore originally took a 122m BB axle, but without the granny and spacers, it fits perfect on my Dura-ace 112mm BB. Shimano cranks shouldn't be put on Campy BBs though since the axle tapers don't match. For most stuff, I use a 13-21 FW but change to 13-23 for stuff >~12%.

edit: Another option is a new freewheel - I just found out that Shimano now makes the 7-speed FW in 11-28! http://harriscyclery.net/site/itemdetails.cfm?ID=1724