Often they’re afraid they’re being recorded. So the conversation should be more like asking for unrelated advice and then making some type of friendship, maybe asking for their contact if you needed any more clarification, etc. Also, they might have a trust network, like you get to them through their friends. So you should ask your friends if they know someone working in this department etc, and get an intro. That way they feel safer. You don’t have to pay them monthly, but yeah, do occasionally compensate them for no reason.

Regarding money, you should start with cash. Have an amount you think is fair on your pocket. Tell them this is what you have and thank for their help. If they don’t look happy or they reject/ask for more, tell them it though, but that you need check if you have more in your car. If the difference is a lot, say you need to ask friend for help. Go back to your car, pretend to talk to someone and come back with more. If they really want a lot more, ask for they’re bank account and tell them you’ll need to transfer (throughout make sure to show them that they asking for a lot and this is very hard for you, you have kids, need to eat, get a cab, pay a loan, etc). Also if you tell them you’ll transfer from a different bank than theirs, they might settle on the cash because interbank transfers are not same day.

All this varies by country and what methods of payment are available.