Hi Slim,
I have stayed in Amsterdam many times and ultimately have found that the way to get good accommodation a bit cheaper is to wait until you arrive and then phone 0031205207000. They can advise you about what hotel rooms are available and you can inevitably get a 4 or 5 star hotel for the price of a 3 star - just because it is last minute.
Doing this used to freak me out in case we didn't get somewhere but we have never not been able to get somewhere and it has always been at least 4 star (and definately under �100 per night).
It is worth deciding what part of town you want to stay in before you go. Near Dam Square is central (and staggerable!!) but a bit rough late at night in my opinion - a few dodgy pick-pocket types about. I usually try to stay in Leidseplein which has a selection of pubs and eateries. The American hotel is quite iconic and worth a visit if you are offered it at the right price.
I hope you love Amsterdam as much as I do.