Why auto forward? With the BB dataplan as someone else mentioned, you get a BIS account with ATT. Log into that and then you can add up to 10 email accounts and automatically get your email sent via RIM every 15 minutes or so.
Works great. Lasts a long time.
As for cost an ATT BB dataplan is $34.99 or $29.99 if you have a qualifying voice plan - at least 450 minutes I think it is.