Not sure if they use this for "total purchase price" for determining eligibility for the plug-in electric vehicle excise tax credit but this is what the Code of Maryland Regulations (COMAR) 11.15.33.04 says about the "taxable price of a vehicle."
B. The taxable price of the vehicle shall include:
(1) The retail purchase price;
(2) The shipping or freight charges;
(3) After manufacture items included with th
e vehicle at time of purchase; and
(4) Manufacturer rebates.
D. A dealer may not include the cost for
any nontaxable items when determining the
taxable price of the vehicle. Nontaxable items include:
(1) Dealer trade-in allowance;
(2) Dealer discounts or rebates;
(3) Extended warranties;
(4) Mechanical repair contracts;
(5) Federal excise tax;
(6) Electronic registration fee; and
(7) Equipment installed to accommodate a disabled person.
http://www.mdauto.org/admin/publications/COMAR-TradeInAllowance.pdf