data bundle too expensive

i bought R1O , i tried 2 buy 100mb , it was not succeessful becos i was left wit R9 because vodacom took their R1 i was owing from airtime advance .my data purchase was unsuccessful since i was left with R9, I didnt recieve any message to notify me that R1 was debited from my account or to say your data purchase was not successsful becos you do have enough airtime of which i had R9.i connected to the internet with my modern , i opened the gmail web page and log to gmail to acceess my emails , after inserting my username and pin my airtime was depleted . i was run out of airtime .i khow using airtime for internet is costy , i feel like voda**** charged me R9 PER MEGABYES OR R9 per mb.this is overcharging , vodacom is robbing loyal customers , am very sad