The short answer is, yes, there are kits available for $600-800 and yes, you can build a nice firearm from them. Check Sitting Fox, Tennessee Valley Muzzleloading, Tennessee Valley Manufacturing, Track of the Wolf, Pecatonica, etc.. None of those are going to be of the same high quality and perfect detail that a Kibler kit will, but there's something to be said for having more options and for creating your own firearm based on your vision and handiwork (versus putting together a kit that was made on a CNC machine and which is all but perfect).
I have a Kibler rifle that I built, I'll surely purchase more in the coming years and have encouraged friends to do the same, but everyone who asks a question these days about a kit is told to "just buy a Kibler". That may be great advice and what's best for some, but it's not universal and it's not the only answer, particularly given the fact that there are only 2 basic styles available today from Kibler. If there were 20 options available and a person suggested that they simply weren't up to building their own rifle, then by all means, my advice would be that if you want a beautiful gun that you'll be happy with, then get a Kibler kit, but at the present time, there are people who may not be looking for that specific SMR or that specific Colonial kit and there are also some who may really wish to "build" their own gun rather than to assemble a set of parts that are all but finished.