If he has an uncapped connection (which it sounds like he does), it will purely be based on speed of the line IMO.
If he has a 384k ADSL line, then 100gb/month is about as much as you can download with it (if you're not pushing it). So with you jumping on it for 5-10gb a month (or maybe more), it might affect his internet more. Specially if you plan on doing torrents (not sure if you're only planning on doing normal email/youtube/www stuff or not)
The higher his line speed the less chance there is of a slowdown (even though if both of you will be doing torrents you can most likely kiss any kind of performance goodbye).
So depending on his line speed, he'll be paying upwards to R599 for 4mbps last I checked to the ISP. I'd say, if he has uncapped, around R2-5 per gb would be sufficient as a per gb price.
Otherwise you can just ditch your dumbass Neotel connection and pay half of his ADSL costs and share the line (even upgrade it if it's not 4mbps)