How hard would it be to get 300-400 extras for free (expense only)?

So I want to shoot a scene in a theatre( which has a capacity of 900) and will have a lot of audience shots but ideally I don't want to use crowd replication vfx for two reasons.

This is for a short film.

1. It's costly for the type of shots I want to shoot. Been asking around and it seems like the avg price is around £1k per static shot.

2. Time The time it would take to set up each shot (if we did want to pay that amount anyway) would probably mean we'd have to both the theatre for an extra day (which would greatly increase the cost.

So I'm thinking it would be actually more beneficial and cheaper to get 300-400 extras, move them around when need be.

However, it's only cheaper if we manage to get them on expenses only. If we paid them minimum wage per hour it would expensive.