[dba-SQLServer]SQL HELP (IIF or IF)

I'm trying to build an SQL Query in Access ADP and could do with some
help

Basically I am trying to build a list that shows qty x cost against a
list of part numbers but also to not add the cost of certain codes

So for instance if I selected P4 using like I would not want it added
into the Tcost2 column

Part	Qty	Cost1	TCost1 Tcost2
P1	2	2.0	4		4
P2	3	3.0	9		9
P3	2	1.0	2		2
P4	1	5.00	5		0	


Is this possible from an SQL query, it is under Access MDB using IIF but
there seems to be no alternative for SQL server.


Cheers

Liam