For most units, they symbol/abbreviation is placed after the number: 200 m, 75° F, 60 mph, 100 watts, etc.
When stating values of money, such as $500, the dollar sign is placed before the number. This is also true of other currency signs like the euro, yuan, pound, etc.
What is the origin of this practice for currency signs?
Edit: As Anixx pointed out, the official ruble abbreviation (RUB) comes after the value. I think this is the same for all official currency abbreviations: USD, CAD, etc. But this just makes it all the more curious: why would the symbol come before the value, but the abbreviation come after the value?
BTW, I consider this an etymology question more than anything, but because it applies not just to English, I wonder if it had some old origin that transcends any one language (possibly in some ancient Greek or Roman practice). This is why I asked it on History.SE.