Prices are so wacky these days, its hard to say. I bought a mint Fender Rumble cab from Guitar Center for $200 this winter, and now the same cabs are going for $350-$450... and they're beat to hell! I did see a F-100 head go for $350 last year.
Many people have shared some incredibly enlightening information there over the years. Any popular forum is going to have a lot of shyte to wade through to find the good stuff.
Do it! Really easy and cheap upgrade. I recently replaced one in a Fender Vaporizer and it sounds so much better than what came stock. I got a long-decay, 3 spring for around $25.