Don't forget there are other costs associated with a build other than the cost of the kit or parts. Some tools you can make (like gouges and scrapers if you are so inclined), but others you can't.
For starters you will need a way to hold your piece while working on it. Most people elect to go with a 2-vice system for full stock LR's because the fore ends are so long. If you are building a half stock then more than likely you can get away with a single vice. Holding small steel or metal parts is different than holding wood, so you'll need a way to deal with that too. Some folks opt for "soft jaws" for their machinists vice(s) to let them handle both wood and metal, and others may use 3 or even 4 vices. It all depends on you.
The other basic thing you'll need is good light and climate control. You can't work very effectively on things you can't see or are too hot or cold to be comfortable. Add in to that mix a place where shavings and filings can fall on the floor and not ruin the place where it's all going on. Some people put all their toys and tools away at the end of each session, but many do not, and leave their tools out till the next session. If you can leave things a little bit messy that will help cut down some of the time on the front and back ends.
Then you can add in the tools and consumables; things like files, rasps, straight edges, knives, measuring devices, drills, bits, taps, pliers, hammers, punches, chisels, gouges, clamps, glue, heat sources, sharpening systems, screw drivers, steel wool, sand paper, finishes, and associated finishing materials. Chances are pretty good that you already have a bunch of this stuff, (screw drivers for instance), but a lot of it is pretty specialized to gun making, or other similar sorts of fine wood and metal work.
Reference materials like books and the internet are their own category, but they can really make things a LOT easier to build something that is historically plausible.
But by far the MOST important thing you need in order to build a gun is the time, and desire to do so. Like most things, you can take this to just about any level you wish. You can spend many thousands on the tools, or get by with just a couple hundred bucks worth.