This seems unwise, because many captcha farms charge less than this. A quick Google search shows one service offering $0.50/1000 challenges. If it's 2x cheaper for an attacker to solve a captcha than it is for a provider to display it, it sounds like the attackers win.
Spammers don't want to hurt the company they attack if they can help it, they need them!
I don't understand why ReCAPTCHA cost so much though. A human solving them is cheaper than a computer/human hybrid creating them?
Regardless of the actual price multiple, it costing anywhere near the price to serve as the price to solve just seems to defeat the point. Really, it costing any money per captcha served just punishes sites that happen to face a higher volume of bots, even if they're a small site. It's just going to push the company to switch to a different captcha service, which may be even cheaper for attackers to solve.