Depends as well on what you mean by "kit". Some of the kits one simply polishes the metal and sands, stains, and finishes a stock. Those are also known as "guns in-the-white". You could go out and shoot them right out of the box in fact. Those are going to have the least advantage over buying a finished gun, unless you are doing your own "dress up" of the basic item.
Then there kits where different amounts of wood working is needed, but the basics, the barrel, the trigger and the lock, are already installed. You save even more money with those, but you have to have the time, and some skill. Depending on where you buy these, you may be upgrading to much higher quality components than you'd get in a factory gun or basic in-the-white factory kit.
Finally, there are simply "parts kits" or "builder's kits". You get the quality parts that you choose, and you are building the item. You can simplify a bit by getting a pre-carved stock, but you are still doing some inletting, and wood working, and the fancier the gun that you want, the more time and skill you will need.
True a few of the factory finished guns you only save a little less than $100 when you buy a "kit". However, for example, a Pedersoli bess is normally $1300, and a kit runs around $1000. I have ordered a "builder's kit" but with the barrel inlet, and a pre-carved stock, and I put the finishing touches on the lock and trigger plus the hardware. Cost me around $700, but the finished product would've cost me around $1200 as it's a plain rifle. Now if I had simply ordered the parts, and had I gone fancier, it would've cost me around $550, and IF I had the time and the skill, it might've been worth $2000 to $2500 when I was done.
LD
