And PowerBI is either free[1] or $10/month per user[2].
Having architected and implemented BI infrastructures using all three, their pricing models all tend to converge to the same ballpark once you get to a standard, fully loaded and integrated installation. But they all have different levers for their pricing sheets, so unique usage models can sometimes take advantage of that to get a substantially better deal. Licensing models that are amenable to minimal/opportunistic usage exist for Tableau and PowerBI, but Looker very deliberately prices out that usage model.
Having architected and implemented BI infrastructures using all three, their pricing models all tend to converge to the same ballpark once you get to a standard, fully loaded and integrated installation. But they all have different levers for their pricing sheets, so unique usage models can sometimes take advantage of that to get a substantially better deal. Licensing models that are amenable to minimal/opportunistic usage exist for Tableau and PowerBI, but Looker very deliberately prices out that usage model.
[1] https://powerbi.microsoft.com/en-us/desktop/
[2] https://powerbi.microsoft.com/en-us/power-bi-pro/