If you want to have different formulas for every supplier here is a workaround
1.Use the wizard to create the formula you want
2.copy the formula from the buy it now field
and add it as additional formula for the supplier you want
Do the same for every supplier you want
when you finish with the suppliers set the general selling formula to 0
If you are using HYDRA you need to edit the item and remove the per item formula which exported from hydra.