Learn Farsi: هر قدر عرضه کاهش می‌یابد، به همان نسبت قیمت بالا می‌رود. - The more the supply decreases, the more the price goes up in proportion.

This structure translates to 'the more... the more proportionately...'. Because this sentence states a general factual rule or cause-and-effect relationship in economics, both verbs (kâhesh mi-yâbad and bâlâ mi-ravad) take the present indicative form, showing that this is a recurring reality.

I know kâhesh means 'decrease', but why use the verb mi-yâbad (from yâftan, to find) instead of kardan or dâdan?

In formal Farsi, kâhesh yâftan (literally 'to find decrease') is the standard intransitive compound verb used when something decreases on its own, like the supply here. If someone were actively decreasing the supply (transitive), you would use kâhesh dâdan instead.

Could I add ke after har qadr in this sentence?

Yes, you can say har qadr ke. Adding ke makes the clause feel a bit more conversational, whereas leaving it out, as seen here, makes the sentence sound slightly more formal and concise, which fits the economic context.

The verbs end in -ad (mi-yâbad, mi-ravad). Is this just the formal version of the spoken -e ending?

Exactly. In everyday spoken Farsi, you would hear mi-yâbe and mi-re. Since this sentence uses formal economic vocabulary like (supply) and (proportion), it adopts a formal written style with the full suffix for the third-person singular.