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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.
Jonesy99- I'm assuming you're in the USA as most users of this site are so I cannot help with chromis costs, sorry! However if you are in the UK like me then you could pick them up for around £4 each. Not expensive at all!