Not sure why the use of paintball. While I have not been part of an urban training program for 7 years, I was impressed with the then state of art simunitions - especially the colored ones that could quickly identify fratricide incidents. You also got a significant reduction in "John Wayne" tactics then you did with MILES - simunitions at least hurt a bit when you got hit.
Unlike normal wax bullets, simunitions are not an inexpensive substitute for live ammunition — costs for simunitions cartridges are as much as three times the cost of live ammunition. Simunitions do, however, provide the most realistic training available.
http://en.wikipedia.org/wiki/Wax_bullets

Besides the rounds, you have to purchase conversion kits for each of the weapons you plan to train with.