You can use your query as a subquery:
select sum(money_earned), group_concat(money_earned)
from (SELECT ((count(*) DIV 20) * 10) AS money_earned
FROM ticket
WHERE closed_by = 269 AND
status = 1 AND
closed_index >= TO_DAYS("2012/01/01") AND closed_index <= TO_DAYS("2013/01/01")
GROUP BY closed_index
) tci;
Because of the rounding, I would be wary of doing this in a single query.