Like Larry says, it depends on the portrait. It can just be 4 easy cuts that take just a few minutes, or many cuts that take hours, from as pattern you spent time making. See below.
I have used frames from the dollar store, and I grab them from wallyworld sometimes for 5 bucks. I think it makes the project look "finished" even if it's a cheapo frame. You can easily recoup the cost of the frame.
Pete