This dates to around 1840 from teh style. The nipples have been replaced.

In the UK you can get a very good quality, clean percussion gun for £2,000, the Purdey name adds a premium because of what Rocketman refers to as Brand Value. This has some wear and repairs - value in the UK is £1,500. You may retail it for £1,750 on a good day. The serial number will date it accurately.