The compact will definitely be cheapest. Perhaps a good place to start would be to compare the gearing you have on your hybrid and see if a compact will get you close- you don't want to invest the time/effort to get a compact and find it's not enough. I'd use the Sheldon Brown gear calculator to compare the gear inches and jumps between gears between you hybrid, your hypothetical setup and your current road bike.

One other point, I like Bostic's setup above as he's running an 11-26 cassette with a triple which will give you many more subtle gearing options on steep/long climbs than a 50/34 with a MTB or Apex cassette.