i always wonder why its so expensive all it does is take off a little health which barely makes a difference to me. i say whats the point but i would like to hear opinions anyone?

Depending on who you battle, Toxic can help by removing lots of HP from Mew/Mysterious Trainer or just a little on Elementary Trainers or Intermediate Trainers.
I believe its damage is calculated in by removing a Percentage of the opponents HP, so Pokemon with lower HP, lose less damage.
Once you get to Mysterious Trainer, Toxic is very helpful. It is almost impossible to beat a Mew without Toxic.