Like has been said, if it has a cut cylinder, $500, but be very sure it is still in good condition as far as timing, and check for cracks in the frame latch, or worse one that has been welded up because of a previous kaboom due to firing 45 ACP factory loads in it. The latch can stretch too.
If it has an original cylinder, anything under $1000 is a good price.
Be aware that Webleys may seem loose to those used to modern American revolvers. Check the cylinder lock up with the trigger all the way back. But don't dry fire it!!! Lower the hammer with your fingers while holding the trigger all the way to the rear.
Sadly, many Webley revolvers have been blown up by firing them with 45 ACP factory loaded ammo.
Also, they have no real forcing cone, so lead projectiles are a must if you reload for it.