Value of car purchase? Please, it's complicated!

ABodyBetty and myself need some help.

Our son called and said he's giving us first dibs to buy his 69 Dart Custom 4 door, 6 cyl. auto. <70,000 miles, seats/interior like new

Value to me? 2K.?

Here's were it's kinda tricky. I was there to test drive and approve purchase of $2200.00 son's money he saved up after wrecking the car I first bought for him.
Test drove it and did the deal, let him give the seller his cash.

Took it home. He has about another $200.00 parts in it.
However, I have approx $1500-$2000 more in parts and 200+ hours in repairs I did while he wasn't around. Signed my half away when he moved out with the stipulation we get dibs if he wants to sell.
You see where I'm driving?
ABB needs a good reliable driver. I replaced everything on this car and it was well cared for when we got it.
We will not be able to recoop our money we have in it unless we buy it back. Then we could sell one of the others.

I thought we could ask you guys to get a fair range for a price keeping in mind the 15-2K$ + labor we already have in the car fixing it up to be safe for him.

Fair price??? Please take a moment to advise. Thanks ahead of time.8)