Ridiculous price changing for motorplan extensions

Approximately 3 months ago i went in to my local dealership to enquire about extending my motorplan for time on my w205 Mercedes Benz C250 AMG line auto. I was given a quote of approximately R21000 and was told that i could extend my plan for this price soon as long as my kilometers do not exceed 60000kms, which is fair. About 2 months later, my car is on 57000kms and i called in to confirm the extension of my plan(time). The salesman gave me a new quote of R43000 which is more than double the price and informed me that there was a price increase in May. He refuses to honour the first quote. How is it possible to change the price to such a ridiculous price when the same product/service is offered. I am highly highly frustrated and disappointed at this. BMW necer ever gave me such issues. Time to sell up and upgrade to a BMW perhaps.