Du kan prova detta:
'$' || Cast((select avg(retail) from cars
where brand = 'FORD' or brand = 'TOYOTA') as decimal(4,2)) as AVG_BRAND_PRICE_01
Om du vill ha mer än $XX.XX t.ex. $XXXXXXXX.XX måste du ställa in decimalen högre t.ex. decimal(9,2)
Exempel SQL Fiddle:http://www.sqlfiddle.com/#!4 /9f684/2/0