For me it's mainly random three ish scenarios of small ships, big ships and all ships.
however I have recently travelled to the outer edge of the civilised space to a to a new hunting site which has a high tec world attached. Now I have only farmed two RES this and one near the starter systems. The latest one has considerably higher bounties often I get Anaconders at 230k where before highest was 120k
unfortunatly there are too many variables that have changed ie ship, rank and system. But it does hint at a base mechanic that effects bounty price.