I don't necessarily calculate or keep track in Quickbooks, but when I buy a piece, I estimate how long I will own it, and how much I will get for it when I offload it it. I calculate (price - resale)/years owned and decide if this is reasonable. If expected resale price is equal to purchase price, cost of ownership is zero, and I buy without hesitation - if I don't like it I resell right away. So for me, buying a $3000 digital piece that will be worth $1000 in two years, is a lot more expensive than buying a $12,000 pair of speakers that I can resell for what I have into them (cost of ownership is zero). This is not hypothetical - I have gone through 4 pairs of speakers without losing a penny on resale (I expect to lose a few grand on my current pair.....). This is how I keep this hobby affordable - don't worry about initial cask outlay, but only about depreciation.
Interesting. I think more about discount to retail. I have a record and receipt of every piece I have ever bought in 20 years and since I always sell or trade-in, I know the total sum spent over 20 years, as well as the % discount to retail across the whole system. My goal of buying below 33 cents on the dollar (67% discount) across the system has meant sometimes searching for literally 5 years to buy one specific piece second hand from a reliable seller.
I always include shipping or travel costs to go pick up a second hand or demo unit, since buying retail usually includes shipping or there isn't any shipping...plus a few newer pieces (isolation) have raised the average up. It also includes the cost of commissions I nearly always pay my local dealer when I buy second hand (even if I found the unit)...the commission is in return for him looking after me if I ever need to have that unit repaired, shipped back to manufacturer, etc...and also to maintain good will with my local dealer who in truth has often brought me some of his best trade-in deals.