(putting on the professorial researcher hat)
The same question occurred to me some time ago so I had to do some digging. According to my research:
In 1966, SF/Fantasy heavyweight Ballantine Books published "Tarnsman of Gor" by John Norman. The first novel in the Gor series introduced a kidnapped and displaced Terran; Tarl Cabot, and the world of Gor -- a world always on the opposite side of the sun from Earth -- a world of slaves and beautiful women, of human domination by the alien, secret Priest-Kings.
1. Tarnsman of Gor (1966) 2. Outlaw of Gor (1967)
3. Priest-Kings of Gor (1968) 4. Nomads of Gor (1969)
5. Assassin of Gor (1970) 6. Raiders of Gor (1971)
7. Captive of Gor (1972) 8. Hunters of Gor (1974)
9. Marauders of Gor (1975) 10. Tribesmen of Gor (1976)
11. Slave Girl of Gor (1977) 12. Beasts of Gor (1978)
13. Explorers of Gor (1979) 14. Fighting Slave of Gor (1980)
15. Rogue of Gor (1981) 16. Guardsman of Gor (1981)
17. Savages of Gor (1982) 18. Blood Brothers of Gor (1982)
19. Kajira Of Gor (1983) 20. Players of Gor (1984)
21. Mercenaries of Gor (1985) 22. Dancer of Gor (1985)
23. Renegades of Gor (1986) 24. Vagabonds of Gor (1987)
25. Magicians of Gor (1988) 26. Witness of Gor (2007)
27. Prize of Gor (2008) 28. Kur of Gor (2009)
29. Swordsmen of Gor (2010) 30. Mariners of Gor (2011)
31. Conspirators of Gor (2012) 32. Smugglers of Gor (2012)
33. Rebels of Gor (2013)
So now there are fewer copies available, but wanted by a few more collectors- but not enough to encourage a lot of reprint runs. So getting them in VG+ or better condition becomes a pricey proposition. I got soo lucky when I purchased one guys' collection; he had some of the later Gor books in VG+ condition. He either didn't know what he had or didn't care. (And I think his wife wanted them gone after the birth of their wee daughter. No foolin')
End of lesson, pop quiz tomorrow...
Bonus Marks for a good Question, but...Because they'd already gone through a few reprint runs as DAW tried to boost sales (before they gave up) they'd only have any real value if they were in primo condition - and even then, maybe ~$10. Sorry. a #1 (Tarnsman of Gor, 1966) 1st/1st in primo condition could go for $75, mostly because it is the 1st, intro's the whole shebang etc
As it happens I have about half of the series available in individual volumes of the harder-to-find original editions. These have been appraised by several paperback collector sources as VHTF (Very Hard To Find), and, in this condition, valued at around triple the listed price. My price is lower because I landed it at a lower price, so we all come ahead.