If you want to go manali only so It won't cost you much amount as recently I was there for 3 night four days alone and my total expenses was around 7500₹ which includes activities also like Paragliding, camping,trekking, skiing,skudo ride and zip line so if you are alone or two people so I would recommend you to take hostel instead of hotel if expense for one person:-
1) Hostel:- 6x400= 2400
2) Bus (delhi to manali and return): 1100x2=2200 (Hrctc would cost less)
3) transport:- Either hire scooty(500-1500) or taxi (2000) (I would recommend scooty)
4) Food:- 600x7= 4200 this is the estimated cost for manali for a week per person