Triggering a workflow off of change in currency field

Hello everyone,

I want to send an e-mail alert if a customers spending drops below a certain amount. I have a rollup field on the accounts form that calculates the spending for the account for the year. Let's say in a year I run update on this field an the account is spending 50% of what they were previously spending is there a way to trigger a workflow to send an alert.

I was thinking of creating a field for the previous years spending and using a workflow to move the data into this field, but is there a way to compare one field value to the other?

I'm trying to figure out the best way to notify the account managers if a customers purchasing amount drops significantly.

Thanks in advance!