Running Balance Or Total ???

I need HELP PLEASE!

I have created a table from importing the data from an Excel spreadsheet. Then I created a simple query using the wizard and adding all the fields that the table had into the new query.

Here is the problem...

I created a couple of expressions using the expression builder for a couple of the fields and I am stuck on how to create an expression for a running balance column.

For example: I have a QTY column (not calculated), a Price 1 column (not calculated), another Price 2 column (calculated with an "If" expression used to look-up a code in one column and if it meets the criteria then it would return a different price than what is in the Price1 column, and a Subtotal column. I need a Balance column (which would start with, for example, $10.00 and then deduct the subtotal amount.

QTY Price1 Price2 Subtotal Balance
- - - - $10.00
1 1.00 - 1.00 $9.00
2 1.00 2.00 $7.00
1 - 2.00 2.00 $5.00

I need very detailed, but very simple directions and fast. I have tried DSum (doesn't work) and every expression I could think of. I need this done in the query I started or in a new query. I don't have Access on this computer, so please sent directions versus a sample database.

Please Help...