When talking about money, the letter K after a number denotes thousands. 1K means $1,000 while 100K stands for $100,000. Both uppercase and lowercase K's are generally accepted and recognized.
The letter always follows the number. Sometimes the dollar sign is included, but it is not necessary. When discussing numbers that do not easily round to a thousand, use a decimal point with one number after the decimal. For example, $12,555 can be expressed as $12.5k.
The letter M is also used when discussing money. "MM" and "M" both stand for "million." Therefore, $30MM is thirty million dollars. When talking about millions of dollars, it is acceptable to use two decimal places ($30.58MM).