I didn't say it was wrong to consider money (after all, freelancers aren't special in exchanging time for money), just that it was not a good idea to have your clients think that you're too focused on the money.

Asking for a bonus might be arrogant, or it might be self-confident. How do you know what your prospective client will think?

The whole bonus idea sounds wrong to me. You're risking a long term relationship for a few points of profit. None of your competitors are doing it - yes, I know that sounds like a bad reason, but is your stand on this means of compensation motivated by what's good for your client?

Bonuses add complexity and misunderstanding to what should be fairly straightforward: Increase your rates as you get better at helping your clients. That's self-confidence.