How much more expensive are hydraulic disk brakes compared to mechanical disk brakes?
It depends on wether you are talking about disc brakes on a drop-bar bike or disk brakes on a flat bar bike. Hydraulic disc brakes with integrated shifters (on drop-bar bikes) are pretty expensive. Generally I made the experience that for the same price you can get a flat bar with hydraulic or a drop bar with mechanical (e.g. Bulls Daily Grinder 1 vs Canyon Commuter 5.0).

On flat-bar bikes the difference seems to be fairly small when comparing hydraulic vs mechanical disc brakes but this varies a lot, since there are also mechanical and hydraulic brakes of vastly different quality.