If you are combining money and literals or variables, and you need results of money type, use money literals or variables:
create table mytable (moneycol money,)
insert into mytable values ($10.00)
select moneycol * $2.5 from mytable
If you are combining money with a float or numeric datatype from column values, use the convert function:
select convert (money, moneycol * percentcol)
from debits, interest
drop table mytable
