Going to type out the full rules as I understand (take with a grain of salt, this could be mistaken!)
In Battle Royale, every shot has a base value of 250,000. This increases by 25,000 each round and can be multiplied. Whenever you make a shot, this value is added to the Royale Jackpot value (after multiplication).
Villains are worth 750,000 for the first shot. This increases by 50,000 each round. After the first shot, they score the normal shot value (250,000 +25,000 per round.) This value is also added to the Royale Jackpot.
Once you hit all villains, the Royale Jackpot lights on the right ramp and all villains except Venom will stop scoring points. You can still score the 250,000+ shot value on all other shots.
When the Royale Jackpot is lit, shooting the right ramp scores the normal shot value of 250,000+, adds it to the Royale Jackpot, and then scores the Royale Jackpot (all of which can be multiplied). This will reset the Royale Jackpot, advance to the next round, and add-a-ball (for the first two rounds). This means the Royale Jackpot is a minimum of 3,250,000 for the first round (750k × 4 + 250k).
edit: Not sure on the red arrow rules… Can anyone confirm?
Completing every red spider shot will add an additional 1,500,000 to your Royale Jackpot. This value increases by 250,000 per round (whether you hit all of the shots or not). You can also get this bonus multiple times in a round if you hit villians for the first time after you complete the red shots. For Doc Ock/Venom, you earn it if you hit them as the last flashing red shot.
Venom may be shot via either the left ramp or side ramp for the first two rounds, but must be hit via the side ramp for the following rounds. (The left ramp will still score the normal shot value.)
The villain shots max out at 1,250,000, and the other shots max out at 500,000.
One additional add-a-ball and 500,000 points are available by completing the Spider Sense targets and rolling over Spider Sense.