Achieving Sum with Variable Rows

Greetings!

Here is a description of my situation:

• In Field 1, the user enters an amount. Let's say $10.
• In Field 2, the user enters a piece of metadata.  The user may then click a button to enter another piece of metadata (another Field 2 displays).  There is only one piece of metadata allowed per line, however there is no limit to how many pieces of metadata may be entered.  Basically, Field 2 is just a repeating subform with a text field in it.
• Field 3 is a calculated field.  This is where I need to place some JavaScript or FormCalc.  What I need is the amount in Field 1 (in this example, $10) to be multiplied by the number of instances of Field 2, however many that may be. 

Any help would be appreciated.

Thank you.