Form element dependencies

I've gotten a bit stuck on something and wanted to see if I could get some help.

I've got a form that has four fields, two of which will be user input. The fields are:

Account Number
Quantity
Size
Price

There are two tables that this will use:

Shirts: Id, Size, Price
Shirt_Order: Id, Shirts_Id, AccountNum, Qty

So the flow is that the customer sees the form and enters their account number and quantity they want to order. They choose the size via a drop down menu and the price will be displayed based on the size chosen in the drop down.

I've got a recordset created for price (text) and size (menu) and they both get populated with data. However I can't figure out how to get the price field to be dependent on what size is chosen.

The other thing is that I need to have the Id from the Shirts table passed to Shirts_Id in the Shirt_Order table. I've added Id to the Size recordset, but I'm not sure if that's the way to go or not.

The last thing I need to do for this page is to display a table, I guess I need a table, at the bottom of the page that will display the rows that have been submitted. Then they can click finish at the bottom of the page to have their order emailed to them.

I'm not sure how to go about doing this part.

This isn't a true commerce site and the orders are going to be exported from the database and put into another system when they are finished. So I haven't gone the route of trying to set it up like a store with a cart, it's basically like an internal site for local YMCA staff to submit shirt orders. But I'm open to suggestions if it looks like I'm doing this the hard way :)

Thanks!