I think you're being a little optimistic there champ. Lightly modded R34 gtt's can be imported for the $26k - $30k mark. All your mods, anything you've done to the car above stock, you wont get back your money for them. People are always going to compare it to the base price of what a stock model sells for, that's just how it is - your car is worth whatever people will pay for it. By all means you can ask for whatever price you think is reasonable, but you need to realise people don't see the same value as you - they just see the extra mods as a bonus - so you'll need to accept the loss if you're planning to sell the car with mods.
All cars depreciate, it's a fact - R34 GTR's can be landed and complied now for mid-$50k. Good luck with the sale. It's a bit cheeky to say you cant find a 34 like yours anywhere. There's a few R34's around with alot more mods than yours, they're just not for sale.