What kind of cage do you have? I know they sell the replacement ramps at 1800PetSupplies.com for really cheap. I bought my external ramp there for about $6.00. That's for a double critter nation. (or single) You might check there also if you don't have a DCN to see if they sell replacement ramps for other cages.
I built a ramp out of a few scrap pieces of untreated wood-- it's pretty easy to do if you've got a drill and some wood scraps laying around. I used half a 2x4 and a little piece at the top for a lip so it can't fall over. Took the girls a bit to get used to it but now they scamper up and down it like champs. Gives them something to chew, too, and I don't worry about them accidentally ingesting plastic.