I want a number in a cell to compound monthly but stay in the same cell from 01-01-2015 Through 12-31-2026

Here is my example:

Sales representative sold a product and is getting commission for it:
They are getting $15 commission from selling the product on 01-01-2015.
When 02-01-2015 rolls around, the cell updates with $30
When 03-01-2015 rolls around, the cell updates with $45
ETC.
ETC.
all the way to 12-31-2026

Cells Referenced (see attached image)

b3 has $15
b4 needs to show $30 on 02-01-2015 automatically(without Human interaction)

See attached.

I want this to happen automatically.. Maybe using the =today() formula???? I am lost for words, and I have researched and talked to many friends about this subject and no one knows how to accomplish this.