Invisalign Retainers or Fixed Wire?

I'm coming to the end of my invisalign treatment and my dentist has advised me to make a decision as to whether I want a fixed wire to keep my teeth in place or retainers which he said I would need to wear for the rest of my life! He said if I opt. for the retainers, then this is included in the price that I paid for the treatment but if I go for the fixed wire, then I need to pay £500. Has anyone else been told this?