Google flights, orbitz, Skyscanner

These websites are pretty efficient to give you an idea of the flight costs I suggest to research using the private mode of your browser or after deleting your cookies because they track your IP and cookies and the price will raise just for you if you don't do that I found these pretty accurate, Skyscanner actually is good only in Europe and the option ' chose all month' to go ' everywhere' from where your at its interesting you 'll get a list of places you can go for cheap during that month, Orbitz it's pretty fine for north America Google flight gives you an idea of the prices overall over the globe, then I suggest to use other engines too and compare all these prices always using private mode.

Google flights