That is correct. Nothing is free. Everything has an opportunity cost. But sometimes there can also be more direct costs like if my bag with 1000 dollars got stolen because I was sleeping in an airport to save 100 dollars or if my back got sore for few days or i caught the flu and was ill for two weeks. Of course there is paradoxically a cost too of going to the hotel (eg my bag can get stolen there too, i could oversleep and miss my flight etc), so it doesn't follow that any option is by default better or 'cheaper'. Depends on the value that we as individuals attach to risk, comfort, stress, health and other intangible costs and our willingness to pay to avoid such.