Without going into REAL specifics, lets just say I have a mysql table called Price with two data, Price A and Price B. 

On my php page, I have Price A displayed by default. What I want to do now is give the visitor the option to display Price B instead of Price A. How can I achieve that? Can anyone help? 

Many thanks

EDIT 
Okay more details: A price is listed on the website. This price is a data and is selected from my mysql table - lets call is HKD. In the same table there is another data called USD. I want to give the option to view the price in USD through a drop down menu. So if they person selects USD, it will retrieve from mysql table the USD data and display it, replacing the original HKD and vice versa when they select HKD again.