but whereas your line rental in concerned, it is to do with the copper they use.
if you opt for the 512 or 1meg package, the copper is "blessed" to a higher degree. dont think of it as the same copper that your neighbour has...remember, it has been blessed, so costs more
In most countries there are different prices for the different speeds, reason being because they include the bandwidth cost into that, so obviously over a faster line you can download more than over a slow one. The difference in line cost will make perfect sense if there was no cap!