You can do it on the cheap. I'm about to do it on the cheap myself! LOL
The problem isn't that you can't do it for less, the problem is that it's SO DAMN EASY to spend money there. There are a million opportunities to spend it on things you WANT, and can't usually find elsewhere. Weed, meals, chocolates, museum admission,souvenirs, etc.
Do what the Dutch do. Just don't spend it.
You just have to be willing to eat and sleep in less that 5 star facilities.
If you don't mind fast food and bunk beds, you're probably in the ball park.
Good weed or hash there is 5 to 20 Euros per gram, so include that in your calculations.
Shop around.
Don't buy all your weed in one place.
The weed is why you're there. Enjoy it.