Your dropdown can handle this task without any scripts. The dropdown can display two values A $2.00, B $5.00 and so on, then when selected use the value from the second field only which would be the amount.
This is not really a scripting task, so much as a question of database structure.
It sounds to me that you ought to have the items and their prices stored in a table, say Products. Then the value list can be set to display the Item Name. I assume that when a user makes a selection this will be in another table, say Purchases, with a relationship between the two based on the productID—which will be the primary field on which the value list is set. Once an item is selected from the list Name and Price fields can be auto-entered by calculation or lookup—you would need to do this so that the name and price are preserved in the purchase record as at the time of purchase.