At least it is not fixed cost/complexity. If you see large number of attacks/spam, you response by increasing complexity (and cpu cycles required to solve it)
Yes, you could change the scale. But the processing power ratio stays the same. At the price per 1k you listed, it's still cost-effective to crack this captcha at 1 per 10s automatically. But noone will accept waiting 100s to post anything.
If you start calculating the hash in the background, that could be useful. I might not want to wait 100 seconds after I've finished my reply, but I don't imagine I'd mind waiting 100 seconds from the time I first opened the thread or started typing.
And you remove CAPTCHA :)