Things may have changed, but back in December 2001, my wife and I paid US$20 each for the R/T backpacker train from Ollanta (AKA Ollantaytambo) to MP. Entry to MP itself was about $20 each. You can get to Ollanta from Cusco in one hour by cab, for about $10 (this last price was current in 2006).

I don't like taking the train all the way from Cusco, because you have to get up at an ungodly hour, plus the train is way too slow when getting out of the Cusco valley. It takes 4 hours to MP. If you take the train from Ollanta, it's only a 90-min train ride.