Simple multiplication formula field

I have 2 fields, a currency field and a number field. I want to multiply the two together, so if the currency is "Proposed Pricing Amount" and the number field is "Billing Frequency", I want a formula field that is

="Proposed Pricing Amount"*Frequency

But I cannot get the formula field to do this simple calculation. My current formula is:

==(${Deals.Billing Frequency per year}*${Deals.Pricing Proposal})

and I am getting this error:

Syntax Error. Check the examples for any functions you're using to see if you formatted them correctly. Make sure your fields are formatted like this: ${Module Name.Field Name}.