Both the Hagerman and Graham Slee are from a Brand that also produces DIY designs.
I see DIY is not broached upon in replies made.
DIY offers options, that in some designs will most likely show the design is easily surpassing some of the Branded Models suggested
A particularvTom Evans Phon' Model, has been publicly scrutinised as a Model marketed at £25K. The same model is proven to be doable as a parts only BOM, using earlier known pricing, for close to £1K.
I would suggest approx' £1.5K if produced as a commission build.
My experiences are showing BOM's of £500 - £1K for a Phon' design to be DIY built are hit and miss, with a few encountered being able to impress.
Same Phon' designs are very easily surpassed in End Sound performance when the BOM for the design are approaching £2K and over.
My experiences had, of a small cross section of the commercially available Branded Phon's offered to market. Where prices come in at under £5K retail, have some very wanting models in the mix, with a few only that contend with a more expensive commercially available model.
In a different context, I have experienced DIY designs at close to £2.5K as the BOM, that have been toe to toe as a competitor to commercially produced £10K + models.