Inserts are the absorbent part of the nappy. Couldn't tell you about the deal without seeing them but that does sound cheap.
Boosters you put into an already aborbant nappy to get it a bit more absorbency. Heavy wetter's, longer journey's, night time ect.
Liners are a thin feel dry fabric usually that let's the wet through to the nappy but keep the poo of to help stop staining and make clean up easier.
If the insert is any good and you LO doesn't pee for England than you should only need one to two inserts per nappy.
I know Blueberry Onesize inserts are popular or Petit Desous (that's is spelt wrong