RCW 82.45.03 tells us what a selling price is:
(1) As used in this chapter, the term "selling price" means the true and fair value of the property conveyed. If property has been conveyed in an arm's length transaction between unrelated persons for a valuable consideration, a rebuttable presumption exists that the selling price is equal to the total consideration paid or contracted to be paid to the transferor, or to another for the transferor's benefit.
(3) As used in this section, "total consideration paid or contracted to be paid" includes money or anything of value, paid or delivered or contracted to be paid or delivered in return for the sale, and shall include the amount of any lien, mortgage, contract indebtedness, or other incumbrance, either given to secure the purchase price, or any part thereof, or remaining unpaid on such property at the time of sale.