50 is pretty unrealistic, especially considering Mega EXs need their regular EX forms in order to be any useful, which technically means you need 100 cards for them to be of any use (unless he just likes collecting them and never plays).
What I mean by a Mega EX needing its regular:
I.e. Mega-Manectric EX needs Manectric EX in order to be of any competitive value, it cannot be played by itself.
I say all of this because I don't know a) what your son wants the cards for and b) how much he knows about the game, like if he just thinks they're cool and likes looking at them or if he wants to play with them. If he doesn't know a lot of about the game, you might want to explain this to him and have him reconsider what he wants.
Having said all that, prices depends on the specific EX, and different EXs have different prices. Cheap EXs can go from 1 USD, and Mega-EXs from ~5 USD. Prices are generally determined by a card's usefulness in battle. The most expensive goes up to about 50 USD per card. You can get a rough idea of prices from this site, and this site actually sells them ; Ebay is also a popular place for selling TCGs.
Also, some EX cards have different art, a regular art (left) and a full art (right) version:
Full arts are more expensive, and there is nothing special about them except collecting value.
I would also like to say that there is a problem with fakes on the market, so be careful when buying them online. I personally have no experience in that, and my only advice is to familiarise yourself with the actual cards you want to buy so you can spot any irregularities (a lot of fakes are generally easy to spot, e.g. they have 999999 HP when the real card should only have 170 HP).
You can try buying some booster packs for your son to open, as they can contain EXs (usually 1 per 4 packs), but it's solely luck based. You'll also get a lot of cards you don't really want. You can also get the theme decks, which have no EXs in them although at least you know what you're getting (the list of cards is on the back of the box). There are also sometimes special promotion tins, which guarantee an EX in the tin (they usually have a little plastic window that shows the EX through the tin), and it comes with booster packs to open.