I have a Marantz SA-11S1 that had the Ultimate Tube mod installed by Chris Johnson of pcX.
I tried to sell it a couple of times over the past few years, but finally gave up and kept it. I still use it as a back up.
The upgrades cost around $3400, but most folks were only willing to pay a couple hundred more than a used stock unit would go for.
Basically, modded units don't have much value on the used market.
This is a generalization, there will always be some exceptions.
Normally though, you will be very lucky to recover 10-20% of the cost of the upgrade.
Good luck.
I tried to sell it a couple of times over the past few years, but finally gave up and kept it. I still use it as a back up.
The upgrades cost around $3400, but most folks were only willing to pay a couple hundred more than a used stock unit would go for.
Basically, modded units don't have much value on the used market.
This is a generalization, there will always be some exceptions.
Normally though, you will be very lucky to recover 10-20% of the cost of the upgrade.
Good luck.