Let me start by saying that I have never owned a production sidelock in either percussion or flintlock...
I bought my first custom made flinter in .45 caliber back in 1977...I was living in Atlanta then, but grew up in eastern NC and used to visit the gunshop in Williamsburg just to see the custom guns that were made there...I was a history buff while in grade school and going to Williamsburg was like Disney World and the State fair all rolled into one...
In college I wanted to get into black powder but the factory guns just didn't appeal to me...
I walked into a gunshop at Stone Mountain, GA and met a guy named Bob Watts...He is one of the gunsmiths written up in Foxfire 5...He handed me one of his rifles that was for sale, asked me to shoulder it with my eyes closed...When I opened my eyes they were looking down the sights...I bought that gun for $470, and have used it ever since...I have also been offered $3,000 for that rifle...
From 1977-1990 I used that gun on squirrels, turkeys and deer...I also shot in several shooting matches and was able to win a few...I rebarreled it to .40 caliber when I finished making my .54 after 2 years in 1990...
Custom barrels usually come with deeper cut rifling than the button rifling in factory guns...The locks are usually of higher quality and spark very reliably and are easy on flints...If made properly they shoulder and balance better than the factory straight barrel guns...Nothing balances better than a swamped barrel on and early Lancaster stock...
The extra cost comes into play because of the higher quality and figured curly maple, the locks such as Siler and Chambers and the swamped barrels...There is much hand work and fitting that goes into a custom gun...I had over 200 hours in my .54 flinter and from what I hear that is about normal for your first build...This doesn't include engraving of the patch box, inlays and relief carving...
Finally, the touch hole is aligned dead center of the pan and the touch hole is behind the heel of the frizzen so priming powder can't plug the hole...When stocked in curly maple they are a work of art...
If price is a problem, get the Lyman, nothing wrong with them as far as I have heard...
In my opinion the customs are worth every penny, the Lyman my even peak your interest even more in this great hobby we are all addicted to...