Tex. Tax Code § 152.002

(a) "Total consideration" means the amount paid or to be paid for a motor vehicle and its accessories attached on or before the sale, without deducting:

(1) the cost of the motor vehicle;

(2) the cost of material, labor or service, interest paid, loss, or any other expense;

(3) the cost of transportation of the motor vehicle before its sale; or

(4) the amount of manufacturers' or importers' excise tax imposed on the motor vehicle by the United States.

(b) "Total consideration" does not include:

(1) a cash discount;

(2) a full cash or credit refund to a customer of the sales price of a motor vehicle returned to the seller;

(3) the amount charged for labor or service rendered in installing, applying, remodeling, or repairing the motor vehicle sold;

(4) a financing, carrying, or service charge or interest on credit extended on a motor vehicle sold under a conditional sale or other deferred payment contract;

(5) the value of a motor vehicle taken by a seller as all or a part of the consideration for sale of another motor vehicle, including any cash payment to the buyer under Section 348.404 or 353.402, Finance Code;

(6) a charge for transportation of the motor vehicle after a sale;

(7) motor vehicle inventory tax; or

(8) an amount made available to the customer under Subchapter G, Chapter 382, Health and Safety Code.

(c) A person who is in the business of selling, renting, or leasing motor vehicles, who obtains the certificate of title to a motor vehicle, and who uses that motor vehicle for business or personal purposes may deduct its fair market value from the total consideration paid for a replacement vehicle if:

(1) the person obtains the certificate of title to the replacement motor vehicle;

(2) the person uses the replacement motor vehicle for business or personal purposes; and

(3) the replaced motor vehicle is offered for sale.

(d) A person who holds a vehicle lessor license under Chapter 2301, Occupations Code, or is specifically not required to obtain a lessor license under Section 2301.254(a) of that code may deduct the fair market value of a replaced motor vehicle that has been leased for longer than 180 days and is titled to another person if:

(1) either person:

(A) holds a beneficial ownership interest in the other person of at least 80 percent; or

(B) acquires all of its vehicles exclusively from franchised dealers whose franchisor shares common ownership with the other person; and

(2) the replaced motor vehicle is offered for sale.

(e) A person who is a motor vehicle owner, is in the business of renting motor vehicles, and holds a permit may deduct the fair market value of a replaced motor vehicle that is titled to another person if:

(1) either person:

(A) holds a beneficial ownership interest in the other person of at least 80 percent; or

(B) acquires all of its vehicles exclusively from franchised dealers whose franchisor shares common ownership with the other person; and

(2) the replaced motor vehicle is offered for sale.

(f) Notwithstanding Subsection (a), the total consideration of a used motor vehicle is the amount on which the tax is computed as provided by Section 152.0412.