People rented out their vacation homes before Airbnb. In the past, you listed it in the classifieds or listed it with a rental agent for that area.

$15/month?

Is it just a shed on some land in the middle of nowhere where all you pay is the property taxes? If not, no, the expenses will be far more than $15/month.

It's the cost home. Property taxes, utilities, maintenance of the property, maintenance of the dwelling, home-owners insurance, etc... In colder climates, heat still needs to stay on or the pipes will freeze and cause flood damage. In warmer climates, air conditioning needs to stay on or moisture will cause mildew and mold.

The big affordability is that the vacation home is often located in a more affordable area than where their primary address is, but not always. Like you mentioned, friends/family can share the cost of the home, it can be revenue generating if they rent it out. Owners might also look at the purchase as an investment if they believe property itself appreciates in value.

Second homes though don't receive the same tax benefits as the primary home. There's also the expense of traveling to and from the vacation home.