How much does a orthodontic retainer cost?

The correction is finished, and I am having a retainer attached to the back.

Maybe it's because something hard or sticky got stuck

Sometimes this falls off.

I go to the dentist to get it put on every time that happens, but

It keeps incurring costs and is also a hassle.

There are also people who say it is better to maintain it for life even after the braces are finished.

How much should I maintain it...?

Is it really a good idea to keep this for the rest of my life...?