I also have the SP10. Bought it from Amazon about 5 years ago for $29 with free shipping. If you can wait and keep an eye on it, the price jumps around quite a bit.
No rust issues, but I store it indoors. I get 5-6 brew days (75 min boil) out of a 20 lb fill. (Or 4-5 with the 17 lb exchange tanks.) Gymrat, sounds like maybe yours needs to be cleaned.