PHP interaction with my SQL data

[fredski]

Please see attached zipped php page, if you look for the section commented <!-- Code for pricing Start --> you will notice I have a form for 4 fields..
Tax Class - products_tax_class_id
Price Net - products_price
Price Gross - products_price_gross
Cost Price - products_price_myself

Each of those has a related database entry, there is also another database entry for supplier percentage - suppliers_percentage.
What I cant figure out how to do is hide the fields for price net and price gross and only display the tax class and cost price. And when a value is entered into the cost price it will fill in the net field based on the percentage increase which will in turn fill in the gross price based on the tax class.
At the moment only the gross price is updated when a value is entered manualy in the net field.

Any help with this would be much apprieated.. Thank you.