Cost compare calculator for web page

[login to view URL] I need to insert a calculator to help online clients compare the cost of a product where they enter the information. I would like to insert it just below the heading of the existing table on the web page linked above. Under the appropriate heading, the client will enter the number of bags (in whole numbers, under the heading "count"), next to that on the right, the price (2 decimal places with $ sign), next to that any shipping costs, if any (with $ sign), next to that calculate the total price (sum the cost and shipping cost, with $ sign), and on the right of that, under cost per bag, calculates the cost per bag (total cost / count = cost per bag to 4 decimal places with $ sign). Inside the fields that are filled in by the client, I would like a hint or help available: ie. In the field box under Count it would say in the box "enter total number of bags here". Under Cost it would say in the field "Enter cost of bags here" and in Shipping it would say "enter total shipping cost here". The wording can be shortened as needed. The field sizes can be adjusted if needed. It can calculate automatically as the fields are filled in. I'm assuming java or javascript, but I am open to whatever works and looks decent. Any additions can be discussed. such as: once a calculation is done,it can display a note saying something like "you're paying $X more per bag than you have to" or "You can save X $$ if you purchase our bags", or something like that.