I think an 11-32 or 34 your less expensive option. I assume it's 11 cogs on the back so all the talk about needing gears with tooth counts close together is left over from the days of 5 speed's...... IMHO of course <grin>. 11 on the back give you plenty of close gearing for the normal range of cycling and leaves you with a couple to use as bail-out gears.
A new shimano 105 cassette only cost me about 30 bucks. You might be SOL though if you have the short cage DR. But a new 105 5800 GS rear DR or newer probably can be found for 34 bucks or less. Changing the chainwheels on might be more expensive if you can find some that fit that crank.