It will generally cost you the price of a boot and you should also buy a custom footbed which will be $150-200. From there usually all work will be free. This is what you are paying for. If you buy a boot online you may save 10-20% upfront but if you need work doing that will quickly overtake that saving.
bootfitting is not really expensive especially when you consider it more a guarantee for comfort.
you may consider custom footbeds expensive but they are the most important part in making any boot fit great so don’t skimp and find someone who really knows how to make them.
custom liners can be good for the right person in the right situation but stock liners are good these days and you are buying them anyway so I’m an advocate for trying them first and if you don’t get along with them you can always swap them out down the road.