I would say that it is an unequivocal fact that Op. 109 is performed (and recorded) considerably less than the ones I listed.
Well, let's try YouTube and Amazon and see what happens:
YouTube search results:
"Beethoven Sonata 28" - approximately 2,900
"Beethoven Sonata 29" - approximately 2,340
"Beethoven Sonata 30" - approximately 2,650
"Beethoven Sonata 31" - approximately 3,040
"Beethoven Sonata 32" - approximately 967
"Beethoven Op. 101" - approximately 496
"Beethoven Op. 106" - approximately 371
"Beethoven Op. 109" - approximately 425
"Beethoven Op. 110" - approximately 492
"Beethoven Op. 111" - approximately 460
The interesting thing here is the very different positioning of Op. 111; first it ends last, but then it ends right in the middle.
I'm also not surprised to see the
Hammerklavier at the bottom of the second list, for I always thought of it as the most neglected late piano sonata by Beethoven.
Amazon.com search results in "Music":
"Beethoven Sonata 28" - 580
"Beethoven Sonata 29" - 282
"Beethoven Sonata 30" - 557
"Beethoven Sonata 31" - 442
"Beethoven Sonata 32" - 533
"Beethoven Op. 101" - 364
"Beethoven Op. 106" - 225
"Beethoven Op. 109" - 474
"Beethoven Op. 110" - 287
"Beethoven Op. 111" - 397