depends how long over, what effect you want it for, and how it has been stored, for example i always buy expired colour from a certain bulk food supply store - they put it on specail like it`s yogurt.$3.95 FOR A 36 frame - that is NZ$3.95 so about US$3.20. In fact they often put it on specail one month prior to expiry, now if only i can convince them that black and white has a growing market and they should stock it.
I have not used any over 12 months expired, but if it has been stored well a little older should be OK.
One area to be wary of is suppliers who offer film without the box and tell you it it was bulk packaged, this way you cannot confirm how old the film is.regards