Back in 2005 or 2006, I bought a 1:18 scale Chevrolet Camaro CONCEPT diecast car for around $30. It is the chrome-colored one with silver rims. It's from Jada Toys DUB City and it is still in it's original packaging. Fast forward to now in 2010, that same Chevy model is now a real car being sold on the market. Would this car now be worth anymore than it was back when I bought it since it is no longer a concept?
Go to eBay. Type in 'jada toys 1:18 camaro concept'. Take a look at the dozen or more listings. They're mainly the bumblebee yellow, but there are a few other ones. Then, look at the completed listings (you have to be a registered user to see sold prices). When they are selling, they're going for under $30, but close to $40 when you add on the cost to the buyer of the shipping.
There aren't any chrome 1:18 models. This just means that there weren't any for sale recently, and none for sale now. It could be just the timing, but it could also be: One, that the model is rare, and the prices on eBay can't reflect its value until one actually gets listed; or two, it's so common that they're not worth listing.
I would lean toward the first, but until one or more actually sell publicly, no one can give you a market value. It's the cost of shipping that kills larger items, because while a buyer may pay you more than what you paid, you have to spend the shipping money, and what you do get for the car has eBay and payment processing fees taken out of it.
You could probably break even on it if you can find a local person that doesn't have to pay for shipping. That is, if you need to sell it. What I would do is sign up for eBay, if you aren't already. You can search for what you want, and then save the search. Type in 'jada 1:18 chrome camaro concept' and you'll be notified by email when one is listed.