The $50 on craigslist looks good. I only buy heater, filter, decors on ebay; they are cheaper.
Tanks I wait for Petco for sale $1/gal. The sale is still going on but for sure will end soon. I got my 20 gal 2 days ago. At that time there are only 2 tanks with that price left.
Or you can look up on craigslist. $50 looks nice but I am sure you can get it even cheaper with all supplies you need; it may take you 2-3 days to find it.
I have 2x tall 20 gal tank. I got them for $20 each with everything: filter, fancy heater, powerfilter, gravel, and decors.
I also got 5 10-gal tanks free and 5 1-gal containers. I got 20 adult bettas and 100+ baby bettas.