Price these days for a late 60s Girard Perregaux HF?

I've decided I have to get a Girard Perregaux Gyromatic HF 36000 bph.

They don't seem to be all that common and asking prices seem to be all over the place. I've read that in the past they have been undervalued, perhaps that has changed?

I'm in the UK so will either have to pay inflated UK prices or take a big tax hit on import...