Koi are only ever worth what someone is willing to pay for them, even the ones with provenance. If your friends fish were worth a great deal then he would already know.
The more vibrant and defined the colours, usually the higher the price. Plain colours are not worth a great deal, black and white fish need to be black and white not grey and white or black and cream, etc.
The defined types sell for higher prices.
Clean markings sell better.
All koi of 24” are worth more than smaller fish but again comes down to what someone is willing to pay for them.
A good idea is to check local fish ads. Here in the uk we have aquarist classified - that is throughout the entire uk so I can see what prices other people are asking (often with photos).
Good to start too high and come down ( you can never go up!)