Fastrack is very expensive, can the Lionel Polar Express be ran completely on a different track that is cheaper, or is this a bad idea?
Fastrack is indeed very expensive. If the cost isn't to your liking, your best bet would be to go to a hobby shop and buy some Lionel O27 track. The last time I bought any, it was under a dollar a section. You need 8 curves to form a circle, and as many straights as you'd like. You also need a lockon, which should be around 50 cents and allows you to connect the transformer to the track.
The hobby shop where you bought your set would most likely be glad to swap out your Fastrack for O27.
You could also go for standard O track, which sits up higher, has heavier rails, and slightly wider curves. The cost is around $2 a section. 8 curves make a circle.
"Lionel trains are the standard of the world" - Jousha Lionel Cowen