Stumbled upon this thread while looking for kit updates and man, I totally feel your pain on those prices! Honestly was in the same boat last month for a huge shoot. Totally worried about the budget after dropping so much on the body itself but still needed power to last all day. @Reply #1 - good point! You really do need that specific power for the crazy top specs. Here is how I saved a ton of cash while keeping my R5II alive:

• Grab a massive power bank instead of buying five official batteries. Grabbed the Anker 737 Power Bank 24000mAh 140W and it basically acts like an infinite power source via USB-C PD while I'm walking between locations.
• If you really need physical spares for your bag, the SmallRig LP-E6NH USB-C Rechargeable Camera Battery is fantastic because you can just plug a phone cable directly into the battery itself to charge!
• Just remember some high-end video features might get grayed out on non-P batteries, but for photos and standard 4K, they work like a charm. Saving that money for glass is totally the right move... Vegas is gonna be amazing with that setup!

Replying quick while I have a sec... honestly man, I'd be really careful with the third party stuff on this specific body. Canon made it so you need that extra juice for the heavy lifting and those new sensors are thirsty.

• Canon LP-E6P Rechargeable Li-Ion Battery Pack is the safe bet. It is the only way to unlock the 8K video and the fastest burst speeds without the camera complaining or limiting features. I would suggest at least two of these for a pro gig.
• SmallRig LP-E6NH USB-C Rechargeable Camera Battery is way cheaper and the USB-C charging is handy, but it will probably throttle your performance. You dont want to be in Vegas and realize you cant shoot the high res stuff because the battery cant output enough power. Make sure to grab at least two originals for the main work and maybe use the cheap ones as emergency backups... better safe than sorry tho.