Depends most on what joinery and cutting tools one feels necessary. Hand tools and a torch make lots of fine custom frames. TIG machine, turning lathe, and mill are high-dollar items. From a few to several thousand bucks if you include those in your needs for those three items--some shops have more than one each of those items.
I have only built three frames but it has been a lot more exspensive than I thought, maybe I'm just a tightwad though. Torch + tanks, large assortment of files, workbench, wood layout table, various hand tools (tubing blocks, height measuring tools, calipers, etc.), tubes, dropouts, silver, lugs, brass, all add up, especially when you do it all at once. I bought everything over a couple of months and I spent a lot of money. I would say that it cost a minimum of 1200-1500 to build my first frame. I has been somewhat less exspensive for the 2nd and 3rd but still not cheap. I haven't started to building for anyone but myself but am saving up to possibly start a small local business next year, saving for the first insurance payment and also trying to save to have the money to buy parts and stuff for the future frames and buildups. I don't really believe in buying junk tools so that may make my cost a little higher but I don't buy top of the line either.
I'm at about $4000 now. Bringheli jig, used vert. mill, shipping to get mill to the shop, files, grinders, Anvil Chainstay fixture (not a necessity, but makes life so much easier, well worth the $700), auto darkening welding mask (again, not really necessary, but auto darkening is worth the coin), hole saws, misc items, TIG rod (10lb min of $18/lb tig rod is an expensive purchase)
but.... my friend is letting me use his shop space, OA rig, Tig welder, compressor... so tack on about $5000 on top of that if I were to do it alone
High quality isnt really the expensive part, Its setting up to do it in a particular way that gets pricey. The base costs are some simple hand tools, and a workstation, and vise; a joinery method, which requires a torch or welder, or both; Frame tools to clean up threads and ream various parts. You are looking at a few hundred to a thousand for the first catagory, 500-1000 for all new in the other two catagories.
I think a mill and a lathe are worth having in a shop, though not necesarry to build a frame. However the combination of metal joining tech, and machining tech, means you can build just about anything else you need from fixtures, to tools, to unique parts for your frames. So it may be worth it to get those items early before you have spent all you money on the things they could build. On the other hand, you can waste a lot of time on those things, so it depends on how soon you need to get going.
I cant believe for a minute that bike builders insurance will cover you. For one thing you may build a few frames and then decide it isnt for you, do you renew the insurance for ever to keep your coverage alive. You need a good insurance, legal, and asset strategy (like being at the lifecycle point where you dont have any) to be safe.