Subject general question: calculated fileds vs performance

Hello! Generally speaking, regarding performance, is "expensive" having calculated fields?

Not complex ones, I mean this type of thing:

... COMPUTED BY (cast(qty * price * tax / 100 as money 2))

So far I don't notice any difference but I'd like to hear opinions about whether is a good idea using them.

Probably having a real field, maintained with a trigger is better? I don't like having redundant data in my database, but I'm note sure what is the better option: waste a bit of space or (probably) decrease performance??