Conditional Rule on opportunity workflow

Hi All,

I'm struggling to get a conditional rule working, basically when an opportunity is Won with a forecast value over £1,000 we want to send an email to a specific user. I thought this would be a simple Javascript to get this working (see below) however, it doesn't seem to be working and sends an email regardless of the value.

Valid =

(oppo_forecast >= '1000.00');

Can anyone see where I've gone wrong, has anyone successfully got a similar conditional rule to work?

For the record I'm using CRM 7.3 Sp3