It depends, if you want to stay in budget hotels then opt for old manali and the average cost per day to be 1k per night. Rest depends on which all places you want to explore
Don't book anything. Just go there and go to the interiors and if possible opt for homestay. not only will it be less expensive but also a very enriching experience.
I think the Budget will be somewhere between 20k to 35k or even higher depending on your accomodation and experience preferences Needless to say the more comfort you want, the more costlier it gets