You can drip acclimate black mollies to full strength saltwater (around $1.99 each), but if you were looking for something truly marine, there are several damselfish species that would be okay (normally in the $4 to $8 range). Unfortunately, most of them are pretty pugnacious for their size. These would be blue damsels, sergeant majors, domino damsels, two stripe damsels, and three strip damsels mostly. Green chromis are in the same price range, but I have not found them to be as hardy myself.
HOWEVER, the better alternative if you are trying to get your nitrogen cycle started would be fishless cycling. You can find a great article about it here:
The (almost) Complete Guide and FAQ to Fishless Cycling
So, I guess the question remains, are you loooking to stock your tank or cycle it?