I'd definitely go for a bike as cheap as possible as the number of bikes stolen every day is pretty high. It really depends on the area you're staying in, but no matter where you are, there's always a chance your bike will get stolen.
There are a few placea to get bikes for not too much money. There's a facebook group for people looking to buy a bike in Amsterdam, lots of cheap bikes on there, you can check that out. Also local bike shops sell used bikes for a lot less than when you're buying a new bike.
There are also less conventional ways like buying a bike for a few bucks from a homeless guy who probably stole the bike, but then again, he just wants to buy food (or drugs) and you just want a bike, so why not help a brother out.
When you finally get your bike, get 2 heavy locks and you'll be fine. Always lock your front wheel, because I've first hand experienced my front wheel being stolen from my bike (yes apparently that's a thing nowadays). Also always try to lock your bike to an object like a street lantern or a fence, so it'll be harder for someone to take your bike.
Best of luck!