The above response is not quite correct, because you have to have a server only with more than 10 seats (although most developers will recommend to have server as soon as you can, because of better reliability and backups). But with your 20 seats you'll need it anyway. With 20 seats you also qualify for volume license, which is slightly cheaper and, being non-profit, you may qualify for non-profit/educational discount. See FileMaker buying FAQ. There's no numbers there though, so you need to request a quote to get an exact price.
The disadvantage of FileMaker is that you'll have to make the database yourself, while the CiviCRM seems to have ready-to-use functionality. FileMaker offers some free templates, but they're normally very simple. FileMaker consultants' fees vary from $20 to $150+ per hour, depending on their class and experience, although many give discounts to non-profits. FileMaker has advantages too, of course; it's generally faster than web apps, as a desktop app it has more familiar interface, decent printing capabilities, etc., but it's difficult to weight these without knowing more details about your needs.