I just bought an Avid mechanical front disc brake for $70 from Pricepoint and a new Ritchey WCS disc front wheel for $65 on Ebay. With the Avid I don't have to buy a new lever and cable so I save money there. Add in about $20 for shipping and my total comes to $155 for a disc brake upgrade for one wheel. Unless you find some POS wheel and brake set, I think that's about as inexpensive as it gets for a disc brake upgrade. And that's only for the front wheel. The back will cost you more.