Gem 65, 66, 67, 68, 69

Typically the higher the grade, the higher the price.

My guess is because of registry sets and centering (eye appeal). It's the same on the coin side.

Usually the 66 will sell for more than the 65.
But, if the 65 has a fancy serial number or better eye appeal, it may sell for more than the 66.

Buy the note, not the holder. GEM is just the term for any note 65 or above.

There are so many factors at work here: Steve says it right - is it a special SN? Otherwise, on a given day in a given auction, a 66 from PCGS or PMG will generally fetch more than a 65, of course. But a month or two later in a similar sale without a 66 competitor, a 65 could well get bid up by a couple of loonies with paddles.

You never know. Tech is right, of course - buy the note, not the holder, but DO pay attention to the paper quality designation or lack of same. A PQ designation may not be the final word, but the lack of one almost certainly is, with the notable exception of early CGA notes which did not denote this.