$20 to each train would probably give you enough RAM for every line in the country. Problem is, we have 58 (plus more on the way) 3 car sets, or 174 cars. That $2480 for the whole fleet you know, plus labor, a lot of money.
There's other reasons why the X'Traps can't run Northern/Caulfield; the cab is in the middle throwing signal sighting out, and they are limited to 90kmh because of computer issues at high speed. This could present a problem for timetabling on the Werribee, Sydenham, Craigiburn, Pakenham, and Cranbourne lines. Mind you, they have better acceleration than the other train which could offset that issue.
It's do-able but I don't think system wide running is that important, more important things need to be done with the system. London Underground has different stock for each line even built to their own line-by-line loading gauge.