I won a santa cruz tallboy frame in a raffle and built the bike out myself. $4K is easy to do for sure. I had to send off a brand new Fox shock to have it rebuilt with a remote lever and that alone was $250. Hell anymore a good fork and wheelset are going to run you $1300+ for the two. I also kept my crank and rear der from my 26er just to save on some money.
Now the $9000 Scott all carbon everything demo bike I rode a month ago was just crazy. Fast fast fast is all I can say about it.