How much does the China post tracking script cost?

It is probably quite trivial, given that the publication in China probably has an API available to obtain such information. Programming something like this is trivial, mainly involves processing the form, publishing the content in the API and analyzing what the API returns. The latter could cause some curses and hair pulling, but it's mostly a quick job. Since there is an API, of course. If not, it's a completely different matter.

Also, if you expect it to be used frequently, you may have to pay a bit for China's publication to use its API (again, if it exists, and again, I have not even checked it remotely, so it could be Free costs money – I have no idea).

Anyone with a little knowledge of PHP should be able to hack this together.