If I play Portable Portal at a battlefield and then react with Legends Jarvan to move a unit to that battlefield, can I pay Portable Portal’s power cost to ready the unit that was moved there?

No — Portable Portal's play trigger has to pick its unit when it's put on the chain, and at that moment the unit Jarvan is about to move isn't there yet.

Both triggers fire off the same event (playing the gear), so you choose their order on the chain, but that ordering doesn't help: to finalize Jarvan's trigger you must already choose the friendly unit, and to finalize the Portal's "pay [C] to ready a unit here" you must already choose a unit at that battlefield. If nothing was there before the gear was played, the Portal's trigger has no legal target and simply never becomes a chain item — the move resolves too late to be readied. Same shape as FAQ #12746 (Jarvan + a trigger that makes a unit).

Two things worth knowing: the Portal's ready does work on a friendly unit that was already at the battlefield beforehand, and Jarvan's ability moves the unit by effect (it doesn't exhaust it), so a ready unit arrives there ready anyway — the readied-to-move case mostly doesn't come up.