Help with an expression to calculate a profit from one of three fi

Using Access 2003 SP3 - in Forms

I know I'm probably asking a very complex question, so a very big THANK YOU
to whomever can help.

I am doing a detailed inventory and need help with an expression in my form
that will calculate my profit based on one of three fields. Only one field
would have the end data.

To clarify what I am working with: Most stock items come by the "unit" and
contain multiple "subunits" which, in turn, contain smaller "pieces."
However, some "units" only come with "subunits" and some only come as "units."

Here is what I have done so far, along with the (format) and followed by the
expression.

Unit Cost (currency) ~ CS:Unit Cost
Units on hand (number) ~ CS:Units on hand
Cost of all Units on hand (currency) ~ =Nz([UnitCost],0)*Nz([UnitsOnHand],0)
Number of Subunits per Unit (number) ~ CS:Number of Subunits per Unit
Subunit Cost (currency) ~
=IIf([NumbSubUnitsPerUnit]=0,Null,Nz([UnitCost])/Nz([NumbSubUnitsPerUnit]))
Subunits on hand (number) ~ CS:Subunits on hand
Cost of all Subunits on hand (currency)
~ =Nz([SubUnitCost])*Nz([SubUnitsOnHand])
Number of Pieces per Subunit (number) ~ CS:Number of Pieces per Subunit
Piece Cost (currency) ~
=IIf([NumbPiecesPerSubUnit]=0,Null,Nz([SubUnitCost])/Nz([NumbPiecesPerSubUnit]))
Pieces on hand (number) ~ CS:Pieces on hand
Cost of all Pieces on hand(currency) ~ =Nz([PieceCost])*Nz([PiecesOnHand])
Total Cost of All on Hand (currency) ~
=Nz([CostUnitsOnHand])+Nz([CostSubUnitsOnHand])+Nz([CostPiecesOnHand])
Selling At (currency) ~ CS:Sell At

Profit (currency) ~ =[SellingAt]-[??cost on hand??]**

**Here is where I need the help. I need an expression that will calculate
the profit based on whichever of the following fields contain the final "cost
on hand" data:

Cost of all Units on hand , Cost of all Subunits on hand , or Cost of all
Pieces on hand

Typically the final cost on hand data will be in "Cost of all Pieces on
hand", but occasionally it will be in one of the other two. Is there an
expression that I can use that will find the last of the three calculated
data fields and use that data?

If there isn't, how would I go about creating a combo box list that would
include only the calculated data in those fields for only the current record?
And if this is possible, could I then use the value displayed in the combo
box to figure the profit?

Again, Thanks ahead of time.
J