Twice upgraded canons 
I'm guessing you guys are selling off shit in the last few seconds of the game to get these scores.
The game sells off everything at the end of the round and adds it to your score automatically.
the keys to high scores are:
*) never ever buy more than you need.
*) maximizing the damage from your towers. Towers that spend the entire time firing are more efficient than those that do not.
*) proper mazing, when appropriate, to force monsters to spend as much time in a tower farm as possible.
*) learning the type of waves so that you can optimize your tower choices. for example, if the natiest waves are hordes of weenies, properly placed inferno towers will do way way way more damage than, say, cannon towers or even gun towers will.