Your minimum budget should be 30,000 for the whole trip. I know this sounds ridiculously low but that's reality. The hostels there are very cheap and you can go couch surfing. It reduces the cost of stay significantly. You can also simply eat some packaged food to reduce the budget of the trip and travel by local transport. You will easily be able to do the trip in this price. Let's break it down, you thought Europe was expensive, but if you budget it right, your trip was never this cheap!!
If you are going for 15 to 20 days, then you can plan for Chez Republic, Greece, Porugal, Switzerland, Germany and many more according to your budget. You can save a lot of money if you travel by Euro Rail. You can get a rail pass for single and multiple journeys. Santorni Island in a Greece is a must visit palace. In Switzerland you can explore Bern, Zurich, Geneva. Munich in Germany will be a good option. If you have schengen visa, you can have hell of options.
Hello Disha, as someone pointed that it would be cheap of you plan right and that's absolutely correct. but 30k even excluding tickets sounds way too less. yes, the hostels and bnbs are very cheap are good place to meet like minded people. though eastern Europe is cheaper and very beautiful, but if you are going to Europe for the first time, starts from the more developed countries. UK, France, Belgium, Netherlands, Scandinavian countries, span to name a few. to connect between.these places check flexi bus. or Ryan Air. if you book in advance and intelligent ly, you will save tons. feel free to msg, if you need specific info.