Does it have the proof marks/matching serial on the barrel, slide & frame with date code?
That is going to be one of the big items right there determining authenticity.
If it is mixed/matched parts and serial it will reduce it greatly.
Is it two tone nickel, k-cote or blued? Nickel brings a premium.
Next I'd look at frame & slide wear. Are they gold to silver in color, that means a lot of wear.
What is the condition of the gun? How are the other wear items?
Is it a CPO gun, refinished, or original condition?
Do you still have the original box and literature?
Any work done to it; SRT, night sights, etc?
If you have all that stuff and its in great shape it would probably bring 12-1500.
A pretty nice p228 will still bring 600-900.
They are one of the nicest balanced pistols out there, and I prefer the folded vs milled slide so long as it doesn't have too much rattle.
Other than a commander sized 1911 I think it is one of the best carry guns ever made.