I am currently using Fido, they're the cheapest plan I know of.

Set-up fees: $35 for the SIM, $35 to set it up, plus taxes, comes to about $80 just to start to use the service on the N900.

Monthly fees: I get the cheapest voice plan, 50 minutes, 50 SMS, for $15. I then add-on 500MB of data for $25.

After taxes, I pay about $45 per month. This gives me full Internet on the N900. I have yet to ever use more than 250 MB in a month.

All that being said, if you can get this Speakout plan working, I'd be very interested. I guess you'd have to SSH to a home PC over port 80 (the aforementioned "port relay") to get full Internet, however...