First is what I call 'Kettling." If you haven't seen the old Kettles movies, look them up.
Next in low cost is checking your lock screws. The strike plate for your dead bolt has screws in it. Take one out. If it is less than 2.5 inches long, replace them with longer screws, at least 2.5 inches long, 3 inches is even better. Talk to your hardware specialist to see if they have a type of harder or less brittle screws. It could be the difference between a bump and a kick to get the door opened.
Check your windows, especially near the doors. Can they be easily broken or entered with a glass cutter, allowing easy access to latches and locks? If so, there are low-cost measures to slowing down the entry process. Ask at your hardware store. They should have a variety of locks or small bars that will help. Room window glass can be reinforced with clear Mylar products that hold the broken glass together to prevent shatter and require another cutting tool to remove the broken glass to make an opening large enough to enter the window. I found some on eBay that we applied to a front window that would have allowed easy access to the front door deadbolt.
Other than a bunch of ammo waiting for anyone who comes inside uninvited, what are your favorite low-cost deterrents?
UPDATE: Great additional tips in the comments! Thanks K and Odessa!!