How can I do this with javascript?

[JayJava]

Imagine a MySQL table that has columns:
Car-------------- color-------------------price
mazda---------------white -----------------$10
jeep------------------blue-------------------$12

I have a PHP form that has a drop down menu (to select CAR) and one input box (for quantity) and button to calculate total price (quantity x price).
When a visitor selects a car I need to picks up value for price from the same MySQL tables.

This is while the visitor is still on the same page (before he click on calculate total button), so it can not be done with PHP. How do I do this with Javascript?
It seems like a simple task but I am very new with js and can not think of a way to access the database without including password and other information for database in the javascript whch can be seen in source code and is a security issue.
any suggestions or help is greatly appreciated.
Thank you in advance
JayJava