The fact that you can pay for both doesn't make them equivalent. To have a similar cost for spammers, you would need to request a challenge that takes many minutes to solve, which you just can't do. There is a strict limit on how long a user will wait for your security check and you can't pretend otherwise.

Let's stop pretending that all things are in the same bucket because "you can pay to have it solved". That's such a weird claim. For the right price you can have someone rob a bank for you, that doesn't mean it's as safe as your $2 padlock.