How to insert a thousand separator (comma) with convert to double

Looking at the standard numeric format strings:

You can most easily use 'N' which will do the right thing based on the user culture, so in your case you can just add "N" as a param to the ToString

([double]12345.67).ToString("N")

12,345.67

For complete custom control, use ... .ToString("#,##0.00") or variations thereof. The . and , will be replaced by culture dependent symbols. In most of europe you'd get 1.234,56.
Another useful picture is 0.0#.

To use a pattern depending on the users (or on a selected) culture, use The Numeric ("N") Format Specifier, as in .ToString("N") or "... {0:N}".