It is unfortunate that some of the client libraries are inefficient, but I'm enjoying my exposure to Amazon's Route53 API.
(This is unrelated to the previous post(s) about operating a DNS service..)
For an idea of scale I host just over 170 zones at the moment.
For the first 25 zones Amazon would charge $0.50 a month, then $0.10 after that. Which would mean:
25 * $0.50 + 150 * $0.10 = $12.50
That seems reasonably .. reasonable.