$18 for the book alone is not out of line though the one you pictured is a bit worn.
As Mars said, it might occasionally be found cheap ($5 is a steal) but not usually.
More likely along the lines of $20-25 for the book alone in excellent condition.
When I first started looking for them several years ago, I paid $25 for a
NM Book and was glad to get it for that too.
Once I got the book I found that I wanted to see more of Tekumel...
And I realised the BOX SET was the goal and was eventually able to get a First Print set for a decent price, though the artwork on the box top is damaged, the contents are complete and all
NM so I was not dissapointed.