I would not recommend camping as you are travelling on bikes. Camping means a lot of extra stuff (tent, sleeping bags, insulation sheets, stove/cylinder, pots etc etc) you will have to carry on bikes all the way from Karachi to Khunjrab. Traveling on bike in mountains is a charm and un-matchable experience, even I enjoy traveling on bike more than my jeep. But carrying this much stuff on bikes will limit your joy of riding and overall travel experience. And its risky too.
Regarding cheap hotels, everything is damn cheap after September. You will easily get a room for 1000 Rs in October in Naran which costs 6000 to 8000 in peak season (June, July, August).
And brother let me tell you if you never been there in autumn season, this is the best time (October, November) of the year to visit this area. Natural beauty is at its peak in Hunza and Ghizer in autumn.
Khair my point was to avoid camping and take advantage of cheap accommodation due to off season.