Formula help

Formula help

I could use some help figuring out a formula field...

${Journeys.Monthly Premium} is a currency field.

If ${Journeys.Monthly Premium} is empty, I want to put TRUE in the formula field.Otherwise, I want to return the ${Journeys.Monthly Premium} in US dollars.

I am using a string field for the formula. Here's what I have for a formula, but it won't return TRUE and I am not sure how to convert the ${Journeys.Monthly Premium} into dollars in the string field. It comes back as just a number.

If(Tostring(${Journeys.Monthly Premium})=='','TRUE','${Journeys.Monthly Premium}')

Can anyone show me where I've gone wrong?