I did this to a bike of mine, while I wasn't to fussy about parts - I upgraded a 24 speed to a 27 speed because it ended up being cheaper (well marginally more expensive short term, but cheaper long term due to other parts wearing out/failing) to replace the entire drive train (chainwheels, cranks, cassette, f/r dérailleurs, chain and shifters) than to get the obsolete replacement part I needed.

It all depends on how much you value the rest of the bike, and what deals you can get on the bits - I got some very good deals of a well known auction site and a rummage (reduced price) bin in a local chain store.