You can get a small Lego set for that amount. My ds loves legos. Also my ds and all the boys in his grade are obsessed with beyblades. If he has siblings to play with, a board game.
How about superhero figures? Legos as others suggested. Computer/video game if you know what system he has, assuming he has one. Remote control car. Construction/building set.
Re: Gift idea for 6 yo boy?
If they are 'mechanical' or like to make things, this Erector set is awesome:
https://www.amazon.com/gp/product/B003HS5JO4/ref=ox_sc_act_title_2?ie=UTF8&m=ATVPDKIKX0DER