At first, you must calculate how much you spend on the needful products for the reception of travelers (land, layout, water, electricity, wifi, etc.).
Once you calculated these expenses, you can keep the amount as a price or if you want to make a slight profit, you can add the price that you estimate as the price of your work.
Keep in mind that, if your accommodation prices are too high, you will reach less participants! Make sure that the asked contribution is in accordance with the services you offer.
Furthermore, this travelers' contribution may, also, take the form of a little help from them or of a purchase of your products.