Adding or subtracting the value of a field in a SELECT

Here's a query I have:

SELECT
    C.CLIENTNUMBER,
    (SELECT SUM(B.FEES) WHERE B.TYPE IN ('1', '2')) AS TOTALFEES
FROM CLIENTS C
INNER JOIN BILLS B ON B.CLIENTNUMBER = C.CLIENTNUMBER
GROUP BY C.CLIENTNUMBER

This is pretty straightforward, but here's the rub. If B.TYPE is either 1 or 2, I want to add B.FEES to TOTALFEES. But if it's 3, I want to subtract B.FEES from TOTALFEES.

Any ideas on how to do this ? Thanks