Try this

cost = IIF(employee's_plan = B4, C4,C5)

Note:
The syntax for the IIF Function is:

IIf(expr, truepart, falsepart)

The IIf function syntax has the following named arguments:

Part Description
exp ----Required. Expression you want to evaluate.
truepart -----Required. Value or expression returned if expr is
True.
falsepart -----Required. Value or expression returned if expr is
False.

IIf always evaluates both truepart and falsepart, even though it
returns only one of them. Because of this, you should watch for
undesirable side effects. For example, if evaluating falsepart results
in a division by zero error, an error occurs even if expr is True.