Adding to another number in a table?

[bloody_football]

I have a form where customers can buy items, when they start buying they start a transaction. What I want to know is if the customer buys another item then how do I add the price onto the total bill in the transaction table?

I have

DoCmd.RunSQL "UPDATE transactions (TotalCost) VALUES ('" & BuyPrice & "' + '" & TotalCost & "')"

I know this is wrong but I am at a total loss on how to do it

 

[Mile-O]

If you have a table of transactions, just use a query to sum the prices in all relevent records to that transaction.