Well, it's not just the 18k+ gold, it's also the time and opportunity cost. The wizard(s) casting the spell has to be there every single day and spend a limited resource. Kind of cuts down on those options to vacation in Aruba if you're the sole caster.
So let's put the real cost at 30k GP. How often is the teleportation circle used? How quickly do you need to make a return on investment? If you need to protect the circle how much does that cost, what's the overhead? A human wizard may want to start making money within a couple decades, an elf could stretch it out to a century or more. Then of course you have to look at returns on alternative investments which could have had compounding interest, etc.
So simplifying again, take that 30k and assume the circle is used once a day. To break even in a year if you include overhead costs it would be something like 100 GP (30k/365=82, then round up) per trip. If you wanted to pay it off after 10 years and took into consideration alternative investment opportunities you didn't sink money into I'd say charging 15 GP sounds about right. Which, to be honest is less than I expected.
But it just depends how often it is used, after all you still need a 5th level spell slot so a 9th level wizard could do it once per day. In any case, that's the basic fuzzy but I think good enough math I would use.