There's an economical way... It's the "megarange/megagear cassette and new rear derailleur" approach. You get a 11-32 or 11-34 cassette (might be a bit hard to find though, 7-speed freewheels are more common in those configurations, you'll probably have to go with a non-Shimano cassette) and a new rear derailleur. You can get an Altus, and Alivio or a Deore all below $25.
Then again, a 12-28 should work just fine with your 1055 RD. Might even take up to 30 big cog, but that's pushing it. Of course, a 11-34 megagear will give you a lot lower first gear
