There are alternative routes you can hike, all of which will be cheaper than the Inca Trail. They're all not as good though...
I didn't hike the trail but you can see where it comes into the city once you're up there. Unlike any other route, you'll be climbing up the back side of the mountain that machu picchu is on...you'll finally make it to the top of the ridge, sweating and exhausted from several 12-hour days of climbing, finally crest over the top and have an awe inspiring view of the city appear beneath you. I really want to go back and do it this way sometime
The alternatives are just treks that end up in Aguas Caliente, the little town at the base of the mountain. Then you'll probably take the bus up like all the other tourists. It isn't all bad, you get to hike in the area but you're not really doing something intimately connected with Machu Picchu. Some of these tours offer more than hiking too, for instance you can mountain bike your way there.
What I did was kind of fun...it is a popular way to get there used only by backpackers. You start in Cusco, take a bus that is not on the beaten track for tourists. Then you get in a collectivo out to a tiny little town that takes you along some pretty terrifying ridges, it is like a near death experience to be racing along 2 feet from a thousand foot cliff for 2 hours. Then you have to hitchhike to a hyrdoelectric plant and hike for a few hours along the train tracks following the Rio Urubamba to get to Aguas Calientes. This trip was probably my favorite thing from my Peru trip because it had the most local flavor of anything I did. Of course it involves only a couple hours of hiking, but if you can't afford the Inca Trail and you're not hung up on hiking it is an awesome way to do it.
Here are some pics from this part of my trip: http://flickr.com/photos/8529845@N05...7601777532125/
I didn't have much from the initial bus parts, I was packed into the minibus like it was the clown car at the circus so I couldn't get out my camera!
Here is a more eloquent blog from a guy who did the same thing. The only difference is that the deadly steel cable platform part is unnecessary now as they've built a bridge. Kind of a bummer actually, I was looking forward to that part...