$350-425 or thereabouts will buy you a Flint KY pistol kit from Track or from TVM. A Kentucky pistol doesn't offer some of the target niceties like sawhandle grip or set triggers that some of Pedersoli's match grade guns offer, but they can turn in respectable scores (mid 90s at 25 yds, low 80s at 50) if the shooter puts his mind to working up a load and learning to hold the gun steady. Maybe could even put in a set trigger.
If your heart isn't dead set on flint, used TC Patriots show up on Gunbroker and similar auction sites from time to time.
And, I believe there's a Yazel flintlock pistol on AuctionArms.com now, still within your stated price range.