I could simply ask the user to type the number in, and then validate that the number is a multiple of 100K and between 100K and 1B, presenting an error message if not. I could strip out any commas that are there as one small improvement.
Are there any ways to improve this? Should I allow the user to enter numbers with a K, M, or B suffix (case insensitive of course) so they could enter numbers like 100K? But then I'd have to provide documentation on how to do that and I'm not sure if I'm really helping?
Should I use a numeric spinner control with appropriate range and step?
Is there a slider control that would work?