Hydro lasts 5 years. VIP runs every year.
As DA Aquamaster mentioned, you can get the new tank around $130~140 including shipping from the online shop or from your LDS sale's rack
Assuming that the new tank price is $140, you have to deduct the shipping and VIP cost at first unless you pick it up locally. So, lets figure, $30 shipping and $10 VIP is $40 (approximation)
So, your bargain for the used tank should start from $100 bucks by checking out the Hydro test date. From this point, the negotiated price is depending on how many Hydro year remains and depreciation cost. Now, it is your turns.
Of course, we are talking about only mint conditioned tank.