I'm running a Stars Without Number game. My players, being cheapskates, would like to rent a starship of some kind. If a normal shuttle costs 200k credits, how would I calculate the cost of renting by the day?
Base it on real world values.
What I did when I was running a space opera campaign (specifically SW, but the principles should apply) is use values you can look up easily and go from there. 200K is conveniently the price of an average home in the US, so you can look online for weekend rental classified ads, AirBNB, etc. In my area that runs 200+ a night (much more for nicer houses; less if it's per person).
Daily rentals of homes are a bit unusual, though. If you want to say shuttles are as ubiquitous as cars in your world, you can compare the cost to rent a car to its total value, and use the same ratio. Funnily enough, that works out about the same as the first method in my area - daily rentals are roughly 1/1000th the price of buying the car new, though it could be half or twice that depending on demand, timing, who's asking, etc. So renting a shuttle from some place without a ton of demand might get you a better deal than the spaceport where everyone is looking to do the same. Finding a reasonably priced rental could be an adventure of its own, or just a roll, depending on how much time you want to spend on this question.