I found this website called Strong Numbers, which is an online price guide for everything. I believe you will find it VERY useful in the pricing of your Hot Wheels collection. Another good purpose is using it before bidding on Hot Wheels on eBay or other auction sites.
Just click on the Strong Numbers link and it will take you directly to the Hot Wheels section. From there choose the name of your car from the drop down menu. Then, select the condition of the car. Finally, click the button at the bottom of the page and there you have it! It's that simple, check it out.