Add price to form field when product selected from drop down box

I have a form which allows me to add more fields. The first field on each form is a drop down box which draws the information from a mysql database. What i need is that when a product is selected from the drop down box that it adds the price to the price field. Then when a quantity is added it calculates the total cost of that item. and updates the total item price field.

I would also like to add a calculate button, that once it is clicked it adds up all of the rows, and then adds the value entered in to the VAT field to generate a total quote price.