Depending upon what you want to do, if you want to do all kind of activities then your cost will go up...if you will do only river rafting like most of the tourists...the charges will vary from 400-600rs per person.
Also the stay is very reasonable in Rishikesh, if you want to enjoy the vibe, I would suggest you to stay with a hostel, Friday will be live music, the ambience is good,the food at the cafe will be good...a decent hostel will cost you around 250rs per day.
Considering stay,food and local transport and sight seeing, It will cost you roughly around 800-1000 rs per day and If you want to enjoy lavishly and experience the best of Rishikesh. It will cost you 1200-1500 per day
I been in rishikesh for month I have stayed at Airbnb which costs around 400 per day, took a scooty on rent which was around 1100 for 3 days . I bought my groceries and cook my food as groceries costed me around 4k per month if I eat at outside it will costs me 100-300 per meal ( I chose cafes near Ganges and street food ) if u go for fancy restaurant then 1 meal could be 500-700 . for 1 month I spent 22450rps which includes ( stay , food , renting bike for 4 times ) if u choose hostels then ur stay can get a stay starting from 200rps or u choose to stay in ashram they will charge from 100 rps to 600 depending on room type u prefer. their are some ashrams who offer free stay for travelers . u need to pay for food or u need to help in ashram . I hope this helps you , have a great journey