For specifying default values in the Modeler, the same format is used regardless of your language. This format uses a dot to separate the whole part from the fractional part. So the value you're looking for is "2000.50" (without the quotes).
This depends on localization and whether group-separation is enabled on the specific fields. In nlNL for instance, 2.000,50 is used, while in enUS, 2,000.50 is the standard. Which to use depends on the languagesetting of the user. For the use of group-separation (dot's or comma's between thousands, you can change the properties of the attributes in the specific forms.
This depends on the default language of the specific project. In nlNL for instance, 2000,50 is used, while in enUS, 2000.50 is the standard. Group separation depends on the property 'Show group separator' in forms.