At this time, you cannot use a greater than or less than search operator for searching number or currency fields on a Directories Search Form. Only exact matches can be returned for the Default Value when searching these fields.
However, you can create a formula field in Salesforce that evaluates the value of a given number or currency field. This could be used as the search field in Soapbox.
For example, if you are implementing a Soapbox front-end portal solution for Volunteers for Salesforce and wish to show all Volunteer Shift records with a value greater than 0 for the # of Volunteers Still Needed field, you could create a formula field named Volunteer Openings that returns the value of TRUE. Volunteer Openings could then be used in the Directories Search Form with a Default Value of TRUE.