I picked up a copy of just the rulebook as well not too long ago in a bulk lot.  There doesn't seem to be too much interest in just the book.  A recent auction sold for $8 with only the one bidder:




** expired/removed eBay auction **

$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.