How do you total the values of the formula fields? I tried "SELECT SUM( formula_field_that_returns_currency ) FROM collection)" but it says there's an error something like "formula_field_that_returns_currency is not found". I tried selecting (not really summing) other fields and it works, but it seems like an error occurs when selecting a formula field.
Formulas are computed on the fly and thus not stored in the database. If you want to be able to do a SELECT SUM(), you might want to use Expressions instead of Formulas.