According to the 'old fashioned' etiquette, if the gentleman - or any party for that matter, extends an invitation for something, unless it's made clear at the time of the invitation otherwise, they are assuming responsibility for expenses
It is polite for the 'guest' to bring a thoughtful gift when it's a host/hostess event (flowers, a plant, wine, etc) or to offer to cover the gratuity(ies) or pay for a meal - or any combination of these extras
So if your gentleman has extended an invitation to travel with him, one way to be perfectly clear that you expect his picking up the tab for the trip is to offer right from the beginning to sponsor an activity. For example: I would love to do this trip, but you must agree right now to let me pay for one nice dinner out along the way. Rather than dinner or together with paying for a dinner you might also say you'll cover the entrance fees to any museum or parks, etc..
This way he'll know you expect his covering most of the expenses - which is as it should be since it was his invitation
Yes. However, your responsibilities in a relationship governed by these classic rules would be to provide home cooked meals now and then, treats of baked goods, picnic lunches, paying gratuities once in a while, modest entrance fees to events, etc..
A percentage guide might help: For example, if he spends 10% of his $1,000 income (obviously I'm keeping this just an example) - then you should spend 10% of your $100 income.
10% is 10% and what is $1000 to him is just $100 to you. Oprah's 10% will be a higher figure than his 10%, but in both cases each person will have invested the same.