subform references in formulas

[Joe Z]

I need some help in subform references in formulas, ie, I have
Main form: frmMain
subform sfrmSub1
sfrmSub1 fields: cost1, cost2, iCnt
sfrmSub1 will have more than 1 record

For instance, how do I do calculations such as cost1 * iCnt, cos2 * iCnt etc. Also, how do I differentiate between sfrmSub1 record 1, 2 etc? Sorry if my terminology is incorrect, I do not do a lot of Access programming!
Thanks

 

[Joe Z]

I have figured out most of the referencing, but I am having trouble with the sum function:

I have tried =sum(sfrmSub1.form!cost1) and get an error. If I use =sfrmSub1.form!cost1 I get the correct value for the first record in the subform. I have also tried sum(nz(sfrmSub1.form!cost1)) and get an error. Any suggestions on how to sum values in a subform. Note, the subform is filtered first with a combobox. baffled.