It is not worth it. Replacing the cable and cleaning the housing is an inadequate compromise - the housing wears more than the cable and has as much or more effect on the shifting.
A worn chain also affects shifting... the chain loses lateral rigidity when worn and you loose more of the side-force from the derailleur in the flex.
Good news, though! If you replace your chain and cassette, and you still think you need a whole new set of parts, you can use the new chain and cassette with all the other parts you buy.