What voltage do electrical fly killers use?

I visited a factory making the 50/60Hz transformer type some years ago. They were several kV (3-4kV) and similar in construction to ballast transformers for neon signs, but without the magnetic shunts to increase the output impedance.

These ones create quite a profound arc which is intended to burn the insect off the grids. Here's a photo of one from Alibaba:

One which I repaired once used rectified 240V mains with a doubler, so about 680V

Here's a teardown video of a cheap plug-in type, which uses voltage doublers to product -630V and +630V across the grid:

https://www.youtube.com/watch?v=LtXmsxRdiUw