The bores are pretty big at 18.8 mm. That makes it a 0.740" bore. I would like to know barrel wall thickness for sure.
What's it worth? Well, most of us would love to own a Purdey hammergun - I know I would. Nice all original ones in proof seem to start at around $10k and go up from there. Given that it's a rebarrel, and one that doesn't match the breach, I would put it at under $5k and maybe around 3500. I don't own any English guns - just like to drool over them. I like to shoot my guns and I would actually prefer one with newer, nicely sleeved steel barrels.
Good luck,
Jeff

"We are men of action. Lies do not become us."
Wesley