That depends on how you count "cost"

If you buy a PC you get a fully ready-to-go hardware unit; if you design & manufacture your own custom microcontroller unit, you must consider costs of designing it and manufacturing it.

With a PC, things like storage, user-interfacing (especially graphical UI), and networking are likely to be much easier; ie, the development cost is likely to be much lower.

With a PC, if it breaks, you can just buy another one; with a custom unit, you will need to consider how you will cope with that...

etc, etc,...

Doesn't mean that a custom unit cannot be a better option - but you need to consider the whole picture.