The cost vs labor... probably better off BUYING an ESC... There are low cost ESC's which will do what you want and save you a LOT of hassle.

Note that most of the common ESC's with BEC circuits have a 8 cell (9.6 v nominal... can withstand short operation at the appx 11.6 v peak for 8 cell NiCd packs) limit on the BEC section... but you can disable the BEC section and then they handle up to 15 v with no problems.

If you check out the discussions RE: Electric flight (and the electric flight discussion forums...) Since they COMMONLY use 20 to 30 cell packs... Someone will know exactly how to make what you want.