PHP MVC model structure
I just have a couple of small points:

Exiting in Constructor

You shouldn't just exit in a constructor, but throw an exception instead. But in this case, the only statement in the try block is an assignment, so there isn't really anything that can go wrong. You should just remove the entire try-catch block.

Naming

Your naming is somewhat inconsistent. For methods you use camelCase, and for parameters you either use all lowercase (which you should always avoid), or under_score. At the very least custpoid should be custpoId. 

I would also use somewhat more descriptive names inside the queries. Having c, cp, cy, as well as ct all in one query is a bit confusing.

SQL Style

Your SQL statements are also a bit inconsistent. If you write SQL keywords in all uppercase, do it always (see for example as, on, and Select).