Porting across OS's is not easy. Especially to a phone, it instantly requires a lot of effort to map everything out to a different control scheme, much less the different resolutions, etc.
It's more effort. Full stop.
Whether that's why theyre pricing it at $5 or something else, it's not your decision. And they don't have to tell you. It's unfortunate it isn't free. But it's ultimately their decision.