My cross' bike is an old reliegh mtb that I put an extra wide BB on, to acomidate striaght cranks and a single chainring(currently a 42 tooth)I still have a casset in back, but hope to turn single speed soon.A straighter fork was put on,and narow tires.I hope to go to drops, and maybe 700c's with it by the end of the season.It weights maybe 25 pounds, proof a mtb bike can be cheap and light and reasonably effective for cross.investment so far:about 150 bucks!