Teaching Is A Sacrifice — Manusmriti 3.70

Teaching is the offering to Brahman (sacred knowledge); water for the ancestors is the offering to forebears; the fire-offering is for the gods; the food-offering is for living beings; honouring guests is the offering to humankind.

अध्यापनं ब्रह्मयज्ञः पितृयज्ञस्तु तर्पणम् । होमो दैवो बलिर्भौतो नृयज्ञोऽतिथिपूजनम् ॥

adhyapanam brahma-yajnah pitri-yajnas tu tarpanam | homo daivo balir bhauto nri-yajno ’tithi-pujanam ||
How to say it: a-dhyaa-PA-nam brah-ma-YAG-nah, pi-tri-YAG-nas tu tar-PA-nam. HO-mo DAI-vo ba-LIR BHAU-to, nri-YAG-no ti-thi-poo-JA-nam.

Teaching is the offering to Brahman (sacred knowledge); water for the ancestors is the offering to forebears; the fire-offering is for the gods; the food-offering is for living beings; honouring guests is the offering to humankind.

These are the panca mahayajnas , the five daily duties of a householder. Notice what teaching is called: a yajna , a sacrifice, an offering placed on the altar. A sacrifice is given away. It is not priced.