I source bargain bits and scour the interwebz for " upgrade orphan" bits. Using a nearly new frame and various bargains I built a £5 k bike for about £2.5 k. Most expensive parts after the frame at £1000 instead of £2500, were the fork, wheelset and seatpost, but they were new, custom built and half RRP respectively. Te drivetrain was preloved and the brakes about half RRP. Bars, saddle and pedals all bought online in special offers and
CRC voucher style bargains.
The joy of a self build is the completely unique build to your own well researched spec. One way to achieve this is to get the frame you want, and scour the online dealers for a bike specc'd the way you want it, and buy it as a donor for your new build. Yu can sell off the bits you don't need, and you can make some serious savings for example by buying last years model in bizarre sizes that haven't sold. CRC for example had a very well specc'd XL women specific scandium MTB which they were flogging off for peanuts. They were snapped up by folk wanting to build custom bikes.
If you want something exotic, it makes sense. If you just want a basic bike you won't save much, if anything at all. What you will get is your own sense of achievement and invaluable experience which will mean you never have to pay for a bike service again.