is the feed algo for warp/fc proprietary, and if so why

Yes

1) no one actually wants "choose your own algo" (stated vs revealed preference)

2) vast majority of people couldn't afford the infra cost if they wanted to run it themselves

Nobody does, they think they want to know but people enjoy things done for them or mystery.

What is an estimate of the costs to run it themselves?

Combat and Degen Veteran -
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Priceless or 1 mil a year... Over a lifetime that's a decent payout 😅

Priceless or 1 mil a year... Over a lifetime that's a decent payout 😅 

https://warpcast.com/zd/0x65f14a08