I've used the Advantage Arms .22lr conversion... works great. I also had an ISSC M22, resembles the G17 in look and feel except for the external manual safety.
If you want REALLY cheap ammo... get an KSC G19 CO2 Blowback airsoft Glock 19. Looks, Feels, simulates recoil, can take real Glock Sights... even breaks down like the real deal.
EVEN CHEAPER AMMO would be just to get a SIRT pistol and a webcam. Then you could download SHOOTOff... a FREEWARE Dry Fire shot tracker (There's also an iPhone app that you can get that does the same thing, but it's $10 I believe). The downside to the SIRT, no movement... slide is stationary, doesn't even rack. Weight is close, feel is closer.
I've setup and used the ShootOFF software for several versions now. You can even have moving and dynamic targets if you set it up with a projector.