Id say the $1.00-$2.50/GB indicates alot more then quality bandwith - though, that is certainly one element.
WRT bandwith - it all comes down to packet loss - or lack thereof - this really differentiates "premium" bandwith.
Other factors to consider are viability of the service you are buying into. The price structure above generally indicates that a company is concerned with long term viability of thier business - they have factored in all costs and left themselves a reasonable margin for profit. This also means they do not need need to grossly oversell their services to be profitable - which ultimately should increase your chances of finding a viable, safe, long term partner.