All of the above reason including the fact that anyone can build their own RPG launcher in any decent mental workshop. Of course, the ammo it self is a whole different story.

Also, RPG 7 is quite light, at 10+kg with round loaded, to carry for long distance ontop of the fact that its damn easy to use. A firer need not have to really aim the damn thing to fire at a target as the aiming system is basically crap piece of metal with 3 or 4 holes to judge your distance.

When USSR changed to the current Russia( I can't remember its exact name ), they needed money badly. So they sold a large among of their arms to the black market, USD$15 can get you an AK-47 at that time. So you can guess what the RPG cost would have been then.
