Bottle fills are different everywhere in the country. Some places it's $20. Some it's $60. It all depends. Running a 55 shot, I was usually able to make about 10-12 1/4 mile passes, which equates to about 2.5 minutes. The FAQ on Zex.com says this...
"Q: How long does a bottle last?
A: It depends on bottle capacity and the size of the nitrous shot being used. On the 75hp setting, a 10 lb. bottle will last around 10 quarter mile runs. On the 125hp setting the bottle will last around 6 runs."