I once knew a low rank cocaine dealer who struggled to get sales and started selling full grams of cocaine for only 20 pounds and it was far better stuff than what you would buy for 40-50 a bag. He sold us a couple of bags for 20, then the second time around he raised his price to 25, then the next time to 30 and now he is trying to push a bag for 40... I never bother to buy anything from him as he wont wake up till around 4pm and he often says he cant come because of whatever reason and then delay the deal till the next day.
I never came across prices being raised by the dealer apart from that one guy. Although I know some people just sell their shit expensive, like a bag of coke of average quality for 50 pounds!!! The same guy charges 30 pounds for a bag of ketamine which is more than the usual 20 quid you pay for the same stuff.
Unfortunately theres not much you can do about it unless you find another dealer. They know that you're gonna keep buying whatever price they ask, coz they know your weaknesses and especially if your're addicted you will go for any deal that first comes across