As I said in my earlier reply (first reply on this thread), I have used researchers for particular problems.
Railway Records can get very complicated, unless you know what you are doing - so I had no problems paying a researcher on TNA's list.
The Jersey records I needed weren't available online; so again worth every penny. I donated a sum to the Channel Islands FHS, too!
Guinness World Records insisted I use a "professional" with qualifications. I don't have formal qualifications in genealogy; just 40 years experience!
Paid for 1 hour, and got our family added to the relevant entry in GWR!
(Most successive generations of twins - we tied the record!

)
For my own part, I am able to research actual documents at the Manx Museum for people.
I always tell people my hourly rate, then ask how much money they want to spend; then I do that amount of research. If they want me to continue, then we come to another agreement.
It's important, for me, to allow the client to be in charge of the amount of money spent!
E.G. I have a client coming from Australia at the end of the month. She wants 1 hour of my time.
But I have done some (free!) preliminary work to ascertain which part of the island her ancestors came from, and in which graveyard(s) some of them are buried.
I would rather have a satisfied client, than fleece them for a lot of cash?
Genealogy is my hobby; for which I now (sometimes) get paid for.
I realise that not everyone is a nice as me?!

Try to use researchers who are recommended. Or at least listed by the relevant archive.
Agree a price, and stick to it.
But also start with some firm leads - the more research you can do yourself, the cheaper it will be.
Finally please remember that what you are paying for is expertise

I can (probably!) find more info in 10 minutes than most people would find in an hour or two
