i've always wondered:

how much costs to airline a re-protection flight to others airlines (partner)?

I mean: suppose you have an e.g. AC flight from LAX to VCE via YYZ and FRA, and assume that AC flight is highly delayed or cancelled, then AC re-protect you on LH flight LAX-FRA-VCE (*A partners):

how much is usually the cost for AC about?

I wrote AC, but this could also apply to other airlines for the same segment typology (C1-C2 or similar segment like C2-C3)!