Is it unethical to teach mindfulness for money?

Yes, it is unethical.

Vinaya Pātimokkha sekhiya Pādukāvagga Navama:

“Formerly, monks, in Benares, the wife of a certain low class man came to be pregnant. Then, monks, this low class woman spoke thus to this low class man: ‘Sir, I am pregnant; I want to eat a mango.’

‘There are no mangoes, it is not the mango season,’ he said.

Now at that time the king had a mango tree with a perpetual crop of fruit. Then, monks, that low class man approached that mango tree; having approached, having climbed up that mango tree, he remained hidden. Then, monks, the king together with the brahmin priest, approached that mango tree; having approached, having sat down on a high seat, he learnt a mantra. Then, monks, it occurred to that low class man:

‘How unrighteous is this king, inasmuch as he learns a mantra, having sat down on a high seat. This brahmin also is unrighteous, inasmuch as he, having sat down on a low seat, teaches a mantra to (someone) sitting on a high seat. I too am unrighteous, I who for the sake of a woman, steal the king’s mangoes. But all this is quite gone,’ (and) he fell down just there.

Neither knows the goal,

neither sees dhamma,

Neither he who teaches the mantra,

nor he who learns according to what is not the rule.

My food is pure conjey

of rice flavoured with meat,

I do not therefore fare on dhamma,

dhamma praised by the noble.

Brahmin, shame on that gain of wealth,

(that) gain of fame;

That conduct (leads) to falling away

or to walking by what is not the rule.

Go forth, great brahmin,

for other creatures boil,

Do not you, following what is not the rule,

from that break like a pot.

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Mahasupina Jataka:

"I saw rancid buttermilk being bartered for precious sandalwood worth a fortune in gold. This was my eleventh dream. What shall come of it?"

"This will happen only in the distant future, when my teaching is waning. In those days, there will be many greedy, shameless bhikkhus, who for the sake of their bellies dare to preach the very words in which I have warned against greed! Because they desert the Truth to gratify their stomachs, and because they sided with sectarians, their preaching will not lead to Nibbana. Their only thought as they preach will be to use fine words and sweet voices to induce lay believers to give them costly robes, delicate food, and every comfort. Others will seat themselves beside the highways, at busy street corners, or at the doors of kings' palaces where they will stoop to preach for money, even for a pittance! Thus these monks will barter away for food, for robes, or for coins, my teaching which leads to liberation from suffering! They will be like those who exchanged precious sandalwood worth a fortune in pure gold for rancid buttermilk. However, you have nothing to fear from this. Tell me your twelfth dream."