Some posts are over-simplifying the issue, others over-complicating it.
Sidewinder by Lee Morgan is available in a new pressing (one title in the Blue Note Classic series) for $20-25. The copies of the album going for hundreds of dollars are different pressings (the Tone Poet series, for one), now out-of-print. Original Blue Note copies from the early-60’s are very expensive as well.
Mobile Fidelity makes an initial pressing of a title, often between 1,000 and 3,000 copies. Some are limited editions (with no second pressing planned), some are not. While still in print (copies available at Music Direct and their dealer network), the MoFi LP’s sell for $34.99. As soon as all the first pressing copies are sold, the price of the copies still out there immediate skyrockets. Some titles will now cost a hundred bucks or more. The same is true of LP titles offered by Analogue Productions.
However, some titles are only temporarily out-of-print, awaiting a repressing. Dylan’s great album Planet Waves has been unavailable for a while, so copies were going for $50-100. It is now again available for $34.99 at Music Direct, a new repress having just been done. The guy still trying to sell his copy on ebay for $50 is apparently unaware of that fact. ;-) Another MoFi title with an upcoming repress is Tapestry by Carole King. Patience, grasshopper.
Analogue Productions did a series of Beach Boys albums, including the infamous Pet Sounds. It was made available in both Mono and Stereo versions, and in both 33-1/3 (a single LP) and 45RPM (2 LP’s) pressings. The Mono version, at both 33-1/3 and 45RPM, has been unavailable since the initial pressing sold out, so of course the price of copies for sale went up. The Mono/45RPM Pet Sounds is now again available from Acoustic Sounds, the 33-1/3 version will be shortly.