Be aware that for purposes of reporting the sale price and for the son's Basis in the property, in most cases the amount you use is the cost AFTER the so-called "gift of equity".  So if the contract says $330,000 with a "gift of equity" of $66,000, the actual sales price for tax purposes is $264,000 (and that is the Basis of the son too).

In the event that $264,000 is lower than the parents' Basis or if Gift Tax was paid, that can alter things, but that is not common.