First thing, when u are low on pd interacting is how u get money.
You should also always have a hunt going, selling shinies earns you
a lot of money. Be careful to choose the Pokemon you're gonna hunt
though. Rumbling is also VERY useful, if your level is high enough
you get gems and boxes and keys and sometimes star pieces which are
very profitable when you sell them. You probably need a high map
level for it through but you gotta start.