The Excel DOLLAR function rounds a supplied number to a specified number of decimal places and then converts this into text, using a currency format. The currency format used is the format that is set as the default on your computer.
The format of the Excel DOLLAR function is :
Where the arguments are as follows :
number    The number to be converted into a text string 
[decimals]   
An optional numerical argument which specifies the number of decimal places to be shown after the decimal point Note :  If [decimals] is omitted, it takes on the default value of 2.  If [decimals] is negative, the supplied number is rounded up to the left of the decimal point. 
Formulas:

Results:

Once a number has been converted, using the Excel Dollar function, it is stored in Excel as text. Therefore, it cannot be used in numeric calculations.
If you want to be able to use the numbers in calculations, you may prefer to keep the values as numbers in Excel and just change the formatting into a currency format. To do this: