Just my opinion here, I know you like the GM manual. The GM accounting manual is GAAP based, it is an accounting requirement for publicly listed companies, and not all of GAAP is accepted by the IRS.
I am a proponent that small businesses should not use GAAP, but should keep their books per the IRS - I mean why pay someone to keep your books per gaap, then pay them to make IRS adjusting entries at the end of the year, and pay them yet again to reverse those adjusting entries for the next year (rhetorical). Or as is more common, keep two sets of books.
The IRS is specific, cost is composed of the purchase price ( or trade in allowance), plus all costs to get it on hand and to get it ready for sale.