It seems like "%100" should be correct if "$1" is correct.
It is the currency symbol that is the odd usage here.
% is a shorthand notation for 'per cent' that has developed since Roman times - it makes for it to follow values (think 100 per cent)
I can't seem to find anything about why so many currency signs precede the value, though.
Different conventions apply to currency symbols and other types of symbol. English contains many conflicting conventions, in many cases because the conflicting conventions were each inherited from a different source. The USA currency prefix position was probably influenced by that of the British currency prefix which I suspect came from the Romans and might reflect Roman conventions or Latin word ordering. The positioning of abbreviations for temperature and other units of measure may have come from other sources.
In short, English is exuberantly inconsistent.
In SI, units follow the number,
- 5 km (Five kilometers)
- 300 K (three hundred Kelvin)
- 999 V (999 volts)
Currency is not an SI unit and so follows a different ISO standard 4217.
% (Percent) is french, meaning if you had 5%, you're actually saying "5 per hundred", as cent means hundred in french. So if you're saying 5 in 100 people are born blind, that's "5 per hundred people are born blind", (5%) where as saying %5 would be "Per hundred 5 people are blind", which makes less sense.