There's no easy answer to that.
You can buy a cheaper ready-made gun for just a few hundred. I would not recommend this course of action.
Rather than go about it from this angle, though, let me try another approach, by describing the progression I went through:
I started out wanting something that look traditional but that closely as possible mimiced the performance of a modern weapon. This was in the early '80s, when the "Jeremiah Johnson"-fueled Hawken craze was approaching its zenith. I became a dutiful follower of the Hawken herd, building an authentic replica of the real thing, which meant it was HEAVY.
I then experimented for years with huge-ass bullets and then on to sabots, all driven by enormous charges of various propellants in an effort to make the thing into a centerfire in performance. I chronographed my loads. I made ballisitc charts and knew how much to hold over to 200 yards. I took some game with it, and it worked well, but it was really no fun at all to shoot. The recoil was horrific, and the muzzle blast was deafening. Worse yet, all this trying to max everything out had resulted in making everything more expensive and more dangerous. Worse still for a guy who hates to clean guns, my over-driven Hawken demanded cleaning between shots and horrendous cleanup at home, using noxious smelling chemicals. Ick.
One day, I was shooting with a guy who pointed out my folly. For all that work and expense and extra risk, I had turned a 130-yard weapon into a 170-yard weapon but at the same time had taken all the fun out of actually shooting it. Of all the game I had killed, none were beyond 110 yards, so, really, I hadn't gained anything at all.
Following his lead, I went to roundballs with loose-ish patches and more realistic charges of real black powder. Suddenly, I could shoot all day, without need to clean during firing except to run a damp patch down the barrel every 10 shots or so. Cleanup at home is simple, too. I put plain cold water in a bucket and use a tube device and a wet patch to suck water into the barrel. I pump it in and out a few times, and it's clean. Simple, easy, and no nasty chemicals. I love that. It still shoots better than I can hold offhand and it doesn't suck to clean, so I noticed I was going out to shoot it more because I was having fun with it. I also shot some elk and deer with it, and they fell over just fine when hit by a 240-ish grain roundball. The paltry number of foot-pounds carried by the balls compared to slugs didn't seem to matter at all. The ball would go in one side of an elk and come out the other side. If there were any vital organs in between, the elk would all seemd to just stand there for about 10-20 seconds as if nothing had happened, and then drop stone dead. Still, that damned Hawken was HEAVY. I got tired of carrying it up and over mountains.
Continuing on that path, toward simplicity, I decided to build a flintlock. I built an old-school one, too, because I wanted a gun that was light and balanced and graceful. In other words, a muzzleloader that was the opposite of my brutish Hawken. The key to all that is to use a barrel that is like those used on the early longrifles:. These are heaviest at the breech, taper toward a waist that is about two thids up the barrel and then gently bell back out at the muzzle. The first one was with a 38-inch barrel. It's impossible to describe how much more balanced and elegant such a weapon feels. It also points and shoots better. Skip the intermediate steps and go right to this point, if you want a flintlock. Also, the heart of a flintlock is the lock itself. Insist on a good one.
Naturally, such a weapon is not to be found at Cabella's for $200.
As a starting point, I will link to Jim Chambers Flintlocks. Jim builds very fine locks and offers kits that are both correct in architecture and a joy to
hold. He offers many styles. If you will hunt with it, an excellent choice is the 38-inch Isaac Haines. For target shooting, go with a 42-inch or 44-inch barrel. If you ever think yiou might hunt with it, get a .54.
http://www.flintlocks.com/rifles02.htm