Budget: About 1$
Materials: An old pair of flip flops and tape.
Recommended for: A hidden blade, backup or a boot knife. Since it lacks a hard core don't try to use it in open fights as a parrying dagger unless you don't care much about your knuckles. It can also be used for a throwing knife with average range. Mind you - since its quite light, don't expect good accuracy and aerodynamics at ranges more than 5 meters.
Here we go then! Mind you - don't pick child size flip flops, unless you want to fight with a pin.
First things first - disassemble the flip flop with the scissors. Its all straight forward from here on in.
Next - measure out how wide you want your knife to be cut out the unnecessary pieces. You should do proper measuring, we don't want it looking too shabby.
Now we are left with our larp knife body(to the left) ready for forming.
Cut out a blade shape (again measure the cuts), if you want - prepare pieces for a guard.
Then wrap the handle with tape, afixing the guard as well if you decided on one(you can get cut it out from the initial shape, you don't have to add it later)