Dollars Earned after running totals roll over

This is a continuation of my earlier question. After some assistance, I was able to correctly return a running total of carpool trips for each employee. Now, I need to add how money the employee earns for the quarter. For each 20 trips, they earn $30, with anything over mod of 20 rolling over to the next quarter. My old method updated the value in a temp table. Here is my current query:

declare @employeeID NVarChar(100), @year Char(4)
Set     @year = '2014'
------
declare @startDate DateTime, @endDate DateTime
Set     @startDate = '1/1/' + @year
Set     @endDate = DateAdd(d,-1,DateAdd(yyyy,1,@startDate))


;WITH cte
     AS (SELECT Datepart(qq, e.eventDate) AS quarterNum,
     CASE DatePart(qq, e.eventDate) 
                WHEN 1 THEN 'Jan-Mar' 
                WHEN 2 THEN 'Apr-Jun' 
                WHEN 3 THEN 'Jul-Sep' 
                WHEN 4 THEN 'Oct-Dec' END AS quarter,
                e.employeeID,
                Sum(t.value) AS trips
         FROM   events e
         LEFT JOIN types t ON t.typeID = e.eventType
         WHERE e.eventType = 'CP' AND Year(eventDate) = @year
         GROUP  BY Datepart(quarter, e.eventDate), e.employeeID)

SELECT a.quarter, a.employeeID, nta.DisplayName AS employee, trips,  
           (SELECT Sum(trips)
            FROM   cte b
            WHERE  a.employeeID = b.employeeID
                   AND a.quarterNum >= b.quarterNum) AS runningTripsTotal,

           0 AS runningEarned

FROM   cte a 
LEFT JOIN SBAIntranet.dbo.NTAuth nta ON 'SBA\' + a.employeeID = nta.AccountName
ORDER BY a.employeeID, a.quarterNum