How far do you want to go? It should at least be a collectible bike.

BMX guys will seek out NOS parts and build up their childhood bike. 3-4k is not uncommon for a nice bike. Retro BMX is in right now 24/26" is hot as 20" is so small. Buy a nice retro bike, sell parts and put top end on. Easy $1500. Build a new retro Hutch, 3k is about right if you use the Hutch parts.

Any road not just cleaned up can be $500 to 1k if you are building decent for what you want, not for value. Road guys are cheaper though for the most part so you wont get much support. Childhood bikes, people go insane for. Here you can an appriasal, there you get scolded for talking value. Lol.