I think that How much do you increase the room rate between off- and peak seasons? would be OK.
If you substituted "price" for "rate" in the example I have given you, I'm sure you would be understood, but I'd recommend "rate", as Language Hound has.
I assume you know that How much do you increase the price? all by itself, with no indication of what price you're talking about, would probably not be understood.
ortak said:Maybe it would be better to use future tense here. 'How much are you going to increase / decrease the price in your new XXX series of I-phone?'
This doesn't sound at all natural to my ear.May be it would be better to use future tense here. 'How much are you going to increase / decrease the price in your new XXX serie of I-phone?'
How much more expensive is the new iPhone going to be?
How much cheaper is the new iPhone going to be?
How much less expensive is the new iPhone going to be?
How much less is the new iPhone going to be?