charging to much bill every time

hey i am using roger mobile phone from last six months and every time they charging me alot of bill & this time given me bill 303 dollars when i call  have to pay all this bill but i am unable to pay this when i ask them cancel my plan then they say u hav to 750 dollars i am depressed know i am mentally harras how can i pay this bill and my actual paln 55 dollars per month. if anybody have solution for this tell me my email is rim27@rediffmail.com.