In Quick Base, unlike excel, you do not need to nest your IFs. Also while extra parentheses don't hurt, they really don't help either.
If(
[Deal Type] = "AMG Sampler", [Down Payment]+[Balance Due],
[Deal Type] = "AMG Platinum Advantage",[CS Down Payment]+[CS Balance Due]-400,
[Deal Type] = "RCI Points",[Down Payment]+[Balance Due]-948)
This can also be written with a case function since you are comparing to the same field repeatedly.
Case([Deal Type],
"AMG Sampler", [Down Payment]+[Balance Due],
"AMG Platinum Advantage",[CS Down Payment]+[CS Balance Due]-400,
"RCI Points",[Down Payment]+[Balance Due]-948)
