You can start at "Puerta del Sol", we can eat in a cheap and delicious place called "100 montaditos". Here is as much small snacks you can imagine, at the most expensive is just 2€. Furthermore, they have an offer that said that in wednesday and sunday, every single product that they sell is just 1€, and on monday, if you ask for a snack and a beer at the same time, the beer costs just 1€, when it should cost 3€. This restaurant is placed in Calle Princesa, but they have lots of it around Spain, so here you can find all they have, and also all the snacks they sell:
http://spain.100montaditos.com/encuentranos/ and
http://spain.100montaditos.com/carta/At night, if the trip is during the weekend are all the discoss that you want to choose which one is the best to go, but if the trip is during the week and you dont know well where to go, then a good option is the disco "Nuit". Here the entrance is free, and inside the drinks priceis 7€, probablythe cheapest that you will find there, because is situated near to the center of Madrid, exactly in street called: Calle Orense, 10. You can find more information here:
http://www.nuitmadrid.es/#!inicio