"There is a reward of one thousand sovereigns"
Sovereign
Creative Commons Attribution Share AlikeSovereign - Credit: Dr Meierhofer

A sovereign was a coin worth about one pound sterling, or twenty shillings, although the coins had no value marked on them. They were first produced in 1489, and are still made today.

If Lyra's world has a currency value similar to the Victorian era, the price on Lyra's head is a large one.