(---This was how I did it in the summer of 2001. Your actual mileage may vary---)
Don't listen to anyone working in Cuzco or especially in Lima. No one there will tell you that this is possible and they all steer you towards the expensive trains for tourists.
For those not doing the Inca Trail, the cheapest way to see Machu Picchu is to
first take the 4 soles bus (3.5 soles=$1) from Cuzco to Ollantaytambo (O-town)
via Urubamba. I recommend staying in O-town at the Hostal Miranda since it is
quiet (good lunches, but eat dinner on the main square).
In Aguas Calientes I bought the $4.50 ticket for the one-way bus to Machu
Picchu in the morning, and I walked back in the afternoon. It wouldn't be such
a Herculean effort to walk both ways, but I thought I'd save my breath for
walking up to Huayna Picchu once I was at Machu Picchu. Besides, if you walk back you can see what Peruvians do with their garbage: they throw it next to the river!
Normal admission is $20, but if you are a student it is $10. Usually you have to be less than 26 years old to use an international student card for Machu Picchu, but I got in for $10 with a card that expired 2 years ago plus I am 37 years old-- and look it! The key was to have exact change and the card in hand. Peru has such a chronic problem with change that if there are people in front of you with big bills, and the ticket guy sees you with a green student card and $10 in your hand, he will take it.
One more thing: there is a $10 Cuzco-area tourist ticket to see all the sights in
the area in a certain number of days. If you buy the other, cheaper ticket just for
a few ruins, keep in mind that you have to see all the ruins in one day, which
would be a very long day as they are very spread out.
And finally, the very pompous: This page © Copyright 2002, Kent Foster