Personally I think it depends on the condition of your tank. If it need repair or you would like to upgrade your reg send it into that tanks company (ex. crossfire = crossfire) they will give you some great deals. I know off the top of my head is for a hydro test and reg swap its $60 plus shipping. If the tanks in good condition and you just want a quick hydro take it somewhere local. Some idea's are your fields store, a shop dealing with fire extinguishers, or diving shop can most likely take care of you. If you get it done local its usually $25-$35. If you send it out its $30+ what ever you want changed and then shipping which is $8-$12 per tank.
"One shot, one kill but then again everyone likes to see the bonus ball!"