in this case you will need to write rules on the dropdowns
on drop down dealer you will need to have three rules (each having three action), one each to set value in each price text box for different dealers selected, and on other three drop downs you will need to have a three rule(having only one action each) to update its respective text box, typical rules will be like
codition if dealer = dealer1
price (textbox system ) = price dealer1 filtered on the condition where system = system
price (textbox racksystem) = price dealer1 filtered on the condition where racksystem= racksystem
price (textbox usb) = price dealer1 filtered on the condition where usb= usb
i would suggest you better change the schema of the table
make only one list
product | dealer | price | category
system prod1 dealer1 100 systems
system prod1 dealer2 200 systems
system prod1 dealer3 300 systems
in this situation rule implementaion will be real easy
on the main drop down you have one rule having three actions
price (textbox system ) = price filtered on the condition where system = category (secondary data connection) and dealer (secondary data connection) = dealer(drop down)