Database Design - Calculating Profits / Earnings

My Database is for a service-based industry. I repair video game consoles.

Relevant Tables:
CustomerT
ConsoleT
WorkOrderT

I'm not using Access' 'relationship' feature but rather am handlign my relationships manually through VisualBasic. For example, my ConsoleT has:

ConsoleID, CustomerID (to link), ConsoleType, SerialNumber, DVDserial, Model, Notes, IsActive

One Customer can have many Consoles. Each Console can have multiple WorkOrders for every time I do work on that particular Console.

I've designed my WorkOrderT to include all the same fields as both CustomerT and ConsoleT. That way all data is preserved in case the Customer say sells his/her console to somebody else. I don't want my database to Cascade change all the WorkOrder data associated with that previous customer. make sense? If not... example:

Peter buys xbox 360 and brings to me to get DVD fixed. I now have a Workorder entry that reflects work done.

Peter sells xbox to Geoff.

Geoff bring same 360 to get Red Ring of Death reballed. I will enter Geoff as a new customer, and make new workorder associated with Geoff's name. The previous DVD drive repair WorkOrder will still stay associated with Peter.

Where should I put information like "Profits", "Income" i.e. Money related stuff. The way I currently have it is that I input the value paid on each invidivual Workorder. It calculates the cost and profit on each Workorder.

Accomplished by putting things like: =([subtotal])*([TaxRate]) in the control Source. Will this kind of thing bog down my database once it starts to get large?

Should ALL calculations stay in queries? I.e. Only do calculations when using a query. Is having a query that calculates things like profits a good way of storing that sort of data? Should I be storing profits/money data into a ProfitsTable separately.