Unfortunately, no explicit rules for the purchasing of normal scabbards, lead lined or otherwise, are given, at least in the SRD. Nonetheless, we can get a fairly precise answer.
An ounce of lead is a costless focus component for the spell Lead Plating found in Kobolds of Golarion. This means that lead is effectively free, at least in the quantities you need to line a scabbard. That just leaves the cost of labor if you aren't willing to do the lining yourself. At this point we have to determine whether a lead-lined scabbard requires a scabbard and lead as raw materials (in which case it costs twice as much as a scabbard does) or whether it requires a scabbard's raw materials (i.e. half a scabbard, in which case the cost is no different than a regular scabbard) to complete.
Assuming you use a normal scabbard, things are still complicated. Scabbards appear to be either free, or possibly included as part of your clothing/weapon/something else, which puts the cost into GM discretion up to twice the cost of the clothes or weapon the scabbard came with.
If your GM is charging you some incredibly unreasonable price for a regular lead-lined scabbard, buy a lead-lined combat scabbard instead, which costs a maximum of 2gp (assuming it's not sharp. Sharp ones go up to 20 gp in the worst case scenario).
If you happen to be playing with the 3rd party Pure Steam Campaign Setting, there are much more explicit rules for lead lining stuff and it is, indeed, free.
If you are the GM, the price should be no different than a normal scabbard, and thus free if you don't track equipment very closely and less than a silver piece but more than a copper piece if you do.