A car you’ve put a ton a work into yourself is hard to estimate in value. I had basically replaced every part in the Fiero, and had a ton good memories autocrossing it, especially the season where Bret and I battled pretty much every race.
Unfortunately your personal history doesn’t add value to a car so you have to consider that when putting a price on it. When I sold the Fiero I got $3500 for it, about $500 more than I paid 4 years earlier. I probably put 2k in parts into since buying it but considering the severe life it had (autocrossing pretty much every other weekend for 4 years) I think the parts investment was reasonable. I still miss that car just because it was the car that introduced me to autocross and was such a blast and a challenge to drive fast.
The GTO on the other hand I have much less emotional attachment to. Partly because I haven’t had it that long, and partly because other than changing the oil and diff fluid I haven’t poured any blood and sweat into it like I did the Fiero. Nothing attaches you to a car like figuring out how to cram the trans back in on a Saturday night only to win your class at autocross the next day by a couple 10ths. I’m sure that will change once the warranty is up on the GTO though and I actually start turning wrenches on it.