There is also the custom of writing two thousand dollars as $2M and three million as $3MM. It varies in several ways from other unit nomenclature.
I beg to disagree, but that is not correct.
Saying the price as Rupees Seventy Five is uncommon, but is indeed the standard when writing out an amount.
Out aloud, “This costs seventy five rupees” is the accepted norm.
Okay, that’s confusing. I occasionally see the MM usage and I always think, "megamillion’. Which would be a trillion.
Regarding the percent sign, it is to the right of a number as it indicates per 100. Not too hard to see how 50/100 evolved to 50%. 
I have never seen that before. I me, $2M is two million, and $3MM is a typo for three million! $2K is two thousand. Where is your nomenclature used? Is it standard in some industries?
It is quite common in business / finance, and is used when writing currency amounts. It originates from the Roman numeral “M”, which means 1,000. I have always found it confusing, too, but when in Rome… 
Huh? I have never seen this before. Anytime I have ever seen $2M written it’s stood for $2 million, as in “NYC apartment sells for $2M.” No one would think that meant $2,000.