I switched my Kona Jake the Snake to a front disc. I swapped the straight steel fork for a carbon fork with disc tabs from nashbar for about $150, then added an Avid BB7 Road Disc.
For me, commuting every day of the year, in the pacific northwest, at 6:30 am in the pitch black, pouring rain, loaded panniers, and steep steep hills, it was the best upgrade I made. Then again the bike itself was worth upgrading. I don't think I would do it to a $300 bike as it would be more econimcal to find a used disc bike instead.