I raise mollies in my sump. I just acclimated FW Mollies to salt over a week or so, and put them in the sump, where there's LR, a couple peppermint shrimp, crabs and snails that take the ride down the drain tanks above.
I use the fry to feed my BTA and Snow Flake Eel. I feed the brooders flakes a couple times a week, and harvest the fry in a trap.
My sump is a 150g rubbermaide stock tank. I put cheato in to give the fry some hiding places.
Cost of the 3 females, 1 male. $5. I have a very small bio-load, so the waste from the brooders actually feed my corals too.