Certainly you can make your stay cheaper by a couple of easy things that people in this thread have already mentioned:

1) Move around on foot or using a citybike (they are free to use but please only within the designated area). Public transport is frequent and safe. Never use taxis. Expensive as hell. I never use them, if I am the one, who has to pay.

2) Do not place calls from hotels. Buy a local SIM-card or use the public payphones.

3) "Cheap" food can be had in Netto and Fakta. Even in the city centre, some restaurants have "cheap" deals.

4) I think 400 DKK is about as cheap as it gets, if you want a hotel room with your own private bath room in Copenhagen.