If your little kid is a junior figurine you would probably know all about these little stuff. Junior action hero collectible figurine start at around $6.99 and could go up to $72.99. There could be newer even more expensive action figures. You must have seen all those little kids who gather around to compare the newest junior action hero collectible figurine that their friends have got.
Generally however, you can get a fairly decent avengers figures within this price range. There are however 9 inch action figures available at roughly the same price and you might even be lucky enough to find a 12 inch one that might not cost all that much more. Maybe such a birthday present might be a very good one that your little son/daughter might value very highly.
Most of the prices on websites are given without adding any shipping costs so you might ultimately end up a paying a little bit more than what you initially thought. Some websites might offer some discounts. It would not matter that your kid already has avengers hot toys action figures as long as you buy him/her an action figure that you haven't already bought.