It depends on the game. (I’m also a PS+ subscriber, so my costs are sometimes lower or even free.)
The PSX games are normally either $6 or $10 and there are some very good games available. There are RPGs like Chrono Cross and all the PSX Final Fantasy games, platformers like the Crash and Spyro games, Castlevania, Parasite Eve and early Resident Evil, and a lot more since I last looked.
They’ve just added what they call PS2 Classics (which is important, as PS3s these days can’t play PS2 games.) Not many yet, nothing I’m interested in, and $10 each.
What they call the Minis (max size 100 MB) are often the types of games you’d find on a smartphone, a cheap computer game, or sometimes a port/remake of an old arcade game or other system. They’ve even got Angry Birds. Again, it depends on the game but most of them are in the $3 to $6 range.
The PSP games generally cannot be played on the PS3. There’s a lot more variety in pricing, anywhere from about $6 to $40. But if he does have a PSP, he could download through the PS3 and load onto the PSP.
There’s what they call PSN Games, some of which are exclusive. More of a grab bag, most are in the $10 range, though a number are $15.
And they’ve just launched full PS3 games available for download. They’re anywhere from $30 to $60 and are multi-gigabyte downloads.
There will also be add-ons. Some of the ones mentioned earlier (like Little Big Planet 2) will have tons. A lot depends on what the add-on is and the game. Often things like weapons, vehicles, skins, and the like are $1-2. Others add maps or other actual content to the point of being whole new games. Those are generally $10 or less.
So one $50 card (basically the cost of a new physical game) would get him several games depending on what he likes.