That format is how it displays on the sheet.
Right click on the field itself in the measures list. Select "Default Properties" and from that pulldown select "Number Format". Set your format there, and it will apply everywhere.
Choose CURRENCY(Custom). You can mess with Ks and Ms for units, mess with the dollar sign (I like to add an extra space after my dollar sign) in the prefix/suffix section.
That's it, thanks!
Joe's already answered your question but one thing you may notice is that the axis label formatting is actually what determines the format in the tooltips. So if you want your axis label to say "$ 300K" then your tooltip will look the same (but be more precise, like $ 371K).
If you're like me, you'd prefer these formatting options to be separated, allowing you to have different formatting for each. There are a few ideas on the subject: