I also found that eBay has some cheaper powerheads and protein skimmers and what not. My local lfs
had cheaper water and sand then PETsMART or some online stores. Live Rock will probably be the biggest cost but u could always start off small and add to that. But I heard more live rock will cycle the tank faster. Just search search search for cheaper prices. But $500 was less than it would've costed when I was researching a 20 gallon nano, my 20 gallon nano even though I'd have a tank and filter, would've been $650 or so. So... Yeah they will have to be willing to go higher. Good luck! It's always great to spend time with the grandparents!