Adding to multiplied values

All

I've been hacking away on a little time billing database and I've been having
difficulty adding to a calculated amount. This is what I am trying to get
to in an insert (for the column 'grandTotal'):

($compRate * $time) + $expenses

I've tried various combinations of quotes, double quotes, parentheses and
what have you, but I still get errors like:

ERROR:  operator is not unique: "unknown" + "unknown"
and
ERROR:  invalid input syntax for type numeric: "7.50 * 5"

I've been able to figure out hours times comp to produce a subtotal, but I
can't seem to pull in an additional amount to produce the grand total.

What's the syntax here?


thanks,

r