I think every priest is different. In our diocese the Mass stipend is $10, but our priests aren’t really sticklers about it. I had a Mass novena offered for a priest ($90), and when I offered the priest money, he turned it away because it was for one of their priests. My friend is also a priest, and I asked him to do a Mass novena for another priest ($90), when I asked him about a stipend amount, he told me, “yeah, I don’t know. You can just send me some money. I just give it away anyways to those people collecting for charity anyways. It’s not like I need the money because I get a salary.” When I was asking our priest for a Mass intention, I forgot to give him money, and just as I was walking out of the sacristy remembered and blurted out, “Oh yeah, you want money!” I was “corrected” and told that he doesn’t want money, and that we’re free to give or not give a stipend. It’s customary to give a stipend, but not required.