The cheapest server at Serverpronto only has 200 GB Traffic
It's suitable if you only need a dedicated server for your site and don't care about traffic
It's not cheap at all if you consider bandwidth usage.
If you consider thats ~2/3mbps of bandwidth, thats in line with what the average hosting server will consume (actually, thats probably more then the average server will consume). I know you may not realise that granted, but if you take aggregate figures from any of the major providers (bandwidth usage versus server count), you'll see the numbers clearly add up to < 1mbps of bandwidth consumption per server.
Given that, I doubt the average (educated) customer would have much issue with 200GB/mo of bandwidth, especially given the price. Its clearly a loss leader to say nothing more.