Oh I agree a disc setup is "lighter" in use but cheaper? Most disc kits are a lot more than a stock drum setup, especially if you already have it on hand. Just trying to stick to the cheapest parameter.

Example, using Dr Diff's site for reference, 10" drum setup complete is $350, cheapest rear disc kit, for drag race use only no E brake which is important if they do safety inspections for registration like in TX, is $509. No weights listed for either. Cheapest rear disc with e brake is $600.

If I had no brakes to begin with I'd probably go with a disc setup in back myself.