We have an only child, so have brought along one of DS's friends on many of our family vacations. Those trips have been of varying degrees of length, distance from home,, and expense. DS has also been a guest on another family's vacation on more than one occasion, so we've experienced the flip side as well. I think each individual scenario warrants its own consideration.
We are taking DS's GF with us on our 3-day vacation to Great Wolf Lodge in Niagara Falls following his graduation this June. We are paying the whole enchilada, including the meal plan. We are covering all accommodations (which include waterpark tickets), transporation and any additional attractions we decide to enjoy while not at the Lodge. We will be shopping at the outlet mall, and her personal purchases there are all we will have her pay for.
We are also taking the GF with us to Orlando this summer. That vacation will be 15 nights/16 days in total....our family's longest trip yet. This is her first time travelling to Florida, so its a big deal for this young lady to say the least. For Christmas, one of DH & I's gifts to her was her ticket to Discovery Cove (she is going to swim with the dolphins!!). That also covers her unlimited admission to SeaWorld & Aquatica. Having her along does not impact our costs of accommodations (we are already in a two bedroom villa that sleeps 8), our rental vehicle (we always get an SUV anyway), or our park-and-fly hotel. We did speak with her mother before extending the invitation for her to join us, and indicated that her parents would be responsible for her airfare and spending money (again, outlet malls galore......she can buy as little or as much as she has the budget for). I also mandatory that they buy her a travel medical package as we are non-US residents (about $35 coverage for our entire stay). Her mom is also buying her Disney waterpark ticket (as we have AP's and don't need to purchase our own admission). So total out of pocket for more than two weeks away....her airfare, and under $100 in waterpark admission and medical insurance.
Our DS was invited to travel with another family from Ontario to Alberta and British Columbia a few summers ago. Again, airfare was our responsibility to cover. Honestly, I can't imagine anyone saying "hey, we'll take your kid with us on vacation half way across the country, and we're happy to pay every last dollar.....even plane tickets!"). They picked up the food, attractions, etc., but that one big cost (because travelling by plane across Canada is much more pricey than travelling by air within the USA, let me tell you!!) was absorbed by us.
So, all that said....for shorter, more inexpensive trips that don't involve flying, yeah....we would probably pay the full amount (souvenirs excepted). For the BIG trips, though, if there is airfare or very expensive attraction tickets to cover, that makes a huge difference in expectations. A beach or camping trip is way different than Disney or Europe!