Could somebody say How much money should I reserve to escrow-expertise of a project?
I don't understand this question, if you are asking about how much money you should reserve for an escrow-expert(service), most of the time 1% of your total payment for your transaction will be enough.
What is the fee for escrow-service if it is provided by well-experienced developer?
Escrows can be developers or not but of course most of the trusted escrows are surely experienced and has built trust from the people here. So deal always with the help of them.