SQL query question

I've been having problems with this query. The second sub-query will not
always return data.
When that is the case, main query will not return a value, even though the
first query does.
Would anyone have any suggestions as to how I could return the
events.EventCost even if
registration.registrationRegCost is null?

SELECT (events.EventCost + registration.registrationRegCost) AS subtotal
FROM (SELECT [Event Registration].RegID AS eventRegID,
SUM(Events.EventCost) AS EventCost
FROM Events INNER JOIN
[Event Registration] ON
Events.ID = [Event Registration].EventID
WHERE [Event Registration].RegID = 399
GROUP BY [Event Registration].RegID) AS events,

(SELECT dbo.[Registration Information].RegID AS
registrationRegID,
dbo.[Registration Types].RegCosts AS registrationRegCost
FROM dbo.[Registration Information],
dbo.[Registration Types]
WHERE dbo.[Registration Information].RegType =
dbo.[Registration Types].RegTypeID
AND dbo.[Registration Information].RegID = @regid2) AS
registration