Wait until Petco has their dollar a gallon sale. Can't go wrong at that price and it will be new so you know it's never been copper treated etc.
I don't know if you'll find one cheaper than $55 because most of them that I see on CL are sold with a bunch of other garbage that you don't need for saltwater and folks seem to be pretty proud of their prices. However, if you want to search multiple craigslists try using s e a r c h t e m p e s t . c o m it will show all the craigslist ads within a certain driving distance. Saves having to look at NRV and Roanoke and Lynchburg separately. Use reef, fish tank and aquarium individually as keywords. Something interesting sometimes pops up, but usually it's a drive (like Hampton Roads or Richmond).