Cheapest answer is get a wide range cassette (i.e. 11-32, or 11-34) and a mountain bike long cage rear derailleur. You'll need a new cassette, new chain, and new rear derailleur. Doable for $150.


You can use a long cage Shimano 105 RD with an 11-32 cassette, although its technically beyond the specifications for the derailleur.