How much is a 1988 Nissan 300ZX · Convertible 2D worth?

The original Nissan Z cars (240, 260/280) are worth good money.

These were made from 1970-77.

The 280ZX from 78 were not as sought after and so a 300ZX is probably not worth much unless in exceptional condition.

I very much doubt it’s worth anything close to $4000.

I used to work at Nissan Motor Corp form 1990-93. this car was made before that time, but we had two of those in the parking lot.. A 1988 300ZX turbo convertible should be worth a good amount of money. Nissan never actually made a convertible version of that car but I believe they were custom done by Straman with approval form Nissan Motors North America. They are pretty rare. Around 150 or so were  ever built....That's it! Nissan dealers would sell them, so it was done properly. Looking at some of the prices that 1980's and 90's 300ZX models have gone for, I wouldn't be surprised if this one wasn't worth more. Your best bet is to check somewhere like Bring A Trailer. That would be the most likely place to find one. It's rare and it's a classic. However, I don't think I'd want to trade a Miata for one, unless you see some personal worth in it. Haven't you learned? Miata is always the answer. 

Between zero and money.

Entirely depending on condition.

It really depends on what type of shape it's in, the mileage, service/owner history, any major collision damage history, etc.

It's too old to have a reliable blue book price. It's value is going to depend on mileage, ownership history, maintenance history, accident history, mechanical condition and cosmetics.

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Determining approximate 

value is not exactly rocket science. I suggest that you go to Auto Trader, Car Gurus, Hemming's and similar websites to perform a nationwide search for 1987-1989 ZX's and see what cars with similar mileage and condition are being advertised for.

It could be a good deal or a great deal.  ("Anonymous" thinks that it is just an 'old car')   The 'great' deal is if it happens to be a turbo engine.  It is possible that it has a lot of rust .  Have that checked out underneath.

$500.00 ....it's an old car.😉

It depends on the mileage.