[throwawayjava]

This. A long-weekend backpacking trip can easily cost between $20 and $100 per person, depending on: distance traveled and size of cars, tolls, number of days on the trail, whether you need perishables such as fuel for cooking and water filters (often weather and trail dependent), meals eaten on the road, trail passes or campground fees, rental fees if you're planning on floating a portion of the trip, etc. Up-front purchases (e.g., backpack, boots) need to be accounted for as well. Also really basic stuff like "do we have enough cash to pay for campsites along the way".

It's a great opportunity to teach basic accounting skills. Simple stuff: summing costs, figuring out which costs are shared by the group and which ones individuals should be responsible for, planning ahead for forms of payment. More advanced: amortization of big expenses across N years of trips (and related cash flow issues), parameterizing cost calculations over weather conditions and terrain, doing all of this in a spreadsheet so that when we do the trip next year we can change some values and know immediately how much it'll cost, etc.