I think it was called 'Priceless'. I read a few chapters but I've had it for a long time on my hard drive and now I can't remember where I have it from.
This is what I remember: Spike waits every morning till Xander goes to work to sleep the day away in Xander's bed. Xander gets fired again but meets some sort of pimp and becomes a hooker. One night Spike smells different men on Xander and wants an explanation.
Ring any bells? Help would be deeply appreciated.