How much does a Invisalign retainer cost?

That's a good question. I don't see why you would not be able to participate in Invisalign's retainer program. The Vivera Retainer Program is about $375-$400 per year and Invisalign continues to ship you retainers 3 or 4 times per year (to prevent a set from wearing out and getting 'grungy'). Check with you orthodontist or dentist to see if you can participate.

It might be possible to use Invisalign's retainer Vivera. However, it would be less expensive to have your Orthodontist make one for you in his own office. That is what I wear. It does fine.

Recent research shows the Invisalign kind of retainer produces better compliance with patients and holds the teeth better than a wire retainer. Plus that kind of retainer will be more effective in preventing grinding at night.

Invisalign offers a retainer service for any patient, whether they were treated with Invisalign or not. These retainers are called Vivera.
For those with Invisalign, any stage of ClinCheck can be used for the retainers. Otherwise, a PVS impression can be sent in. Those who have fixed lingual retainers can also have Vivera retainers made to fit around them.
Vivera is available as one single upper or lower retainer, one complete set, or as a renewable subscription where a new set is available every 3 months. With the subscription ,the first set is delivered to the office and subsequent sets can be sent to the office or mailed directly to the patient.
Though I cannot show any research at this time, my anecdotal experience with this product is that they are more durable that Invisalign trays, and lab-style essix retainers. They are slightly thicker but conform to the teeth very well. If lost, a new impression is not required, since the aligners can be fabricated from a digitally stored model.
The cost is going to vary from office to office and depending on which product you decide to go with. Good luck!