I assume by 'sandbox sand' you mean playsand? (I'm in th UK, lol) If so, yes, it's perfectly safe, as long as it's well washed. It's very cheap, so I wouldn't risk collecting it myself from the beach; apart from the salt you wouldn't know what other pollutants might be lurking in there.
I don't know exactly what it would be in dollars, but I paid appoximately $4 for enough sand (two 15kg bags) to do two 48"x15" tanks, plus half a 36"X12" (although I did use some gravel for the plants, just to hold them down!). You don't need a thick layer.