I have a gigabit connection and the contract specifically states that they guarantee a maximum throughput of 400Mbit at peak times.
Otherwise 0Mbit is consistent with the terms of the contract, which is probably exactly what the ISP was going for.
It says nothing, there are two weasel word exceptions in there.
So it could save you nothing, or it could save you 15%, or even more. Basically we have no idea what it'll save you, if anything.