Bonjour cher Monsieur
I consulted decades ago 1970s..1990s. My recollection is $300..500/hr, no fixed cost is possible.
Nowadays most of the PSU design is in China and consultants are rare and fully booked for years.
The customer writes a SOW...Statement of Work, defining
source, load specs
environnement, temp amb min, max
cooling, airflow, acoustic noise
Regulatory compliance for EMI, power quality and safety
targeted sizes and weight
EAU..estimation of yearly use
targeted production cost
Define insurance, hold harmless and potential liabilities.
A serious OEM firm will be familiar with the use of expert consultants.
Check the IEEE power electronics, and industrial applications proceedings for advertisements...lots of positions available but no ads for consultants...
Finally depending upon the anticipated production quantity, the costs of design, testing, compliance and set-up are amortized thus a 100k quantity products can afford a 200K$ design but a 10..100 pcs run cannot.
Just the ramblings of an old retired EE
Bon chance
Jon