Sorry, but what is the issue?
You can plan on SelPrice and Qty, then you will have script in default.lgf to calculate:
*WHEN ACCOUNT
*IS SelPrice
*REC(EXPRESSION=[ACCOUNT].[SelPrice]*[ACCOUNT].[Qty],ACCOUNT=SalesAmount)
*ENDWHEN
*WHEN ACCOUNT
*IS Qty
*REC(EXPRESSION=[ACCOUNT].[SelPrice]*[ACCOUNT].[Qty],ACCOUNT=SalesAmount)
*ENDWHEN
If you want to restrict this calculation to only some products with property CALCAMOUNT=Y
then add before WHEN:
*XDIM_FILTER Product = [Product].properties("CALCAMOUNT") = "Y"
Vadim