The If Statement can be used as a function in Web TimeSheet custom columns,

If (cond, true, false)

Using this function the following formula can be used to achieve the required output in the report.

If (FullTaskName = "<project name> / <task name>", TotalHours * 3.75, 0)

This condition multiplies the $3.75 with the Total Hours that is recorded only against the given specific task name.