Don't know if you have this figured out yet or not.

Lets break down the expression

Cost == "DRU" ? Values : NULL(DT_I4)

Cost=="DRU" is the expression or if statement if you will.

So if the Cost value for the row your on = "DRU"

? = use the value if True,

: = Use the Value if Fales

So here's what I see when running your sample data.

ServerDRU

SAB245 200

SAB246 2001

SAB247 NULL

SAB248 NULL

SAB249 NULL

SAB250 NULL