Hotels are expensive... if you guys are cool w/ a private room in a hostel, I'd recommend going that way. Otherwise you're looking at a typical going rate of €80-90/night for pretty standard accommodations.
In Paris, though, I'd recommend the Eldorado Hotel (www.eldoradohotel.fr)
, which can be as little as €50/night.
Otherwise, say you burn more than you expected on hotel and wind up with €400 for five nights of toolin' around, you should still do fine. There's plenty to do in Paris and Amsterdam for little or for free.
As Lauren mentioned, a carnet of 10 metro tickets in Paris is €11.50, and if you walk a lot (which is easy to do in Paris), that should work for the both of you for a couple of days. Otherwise, a single ride ticket is €1.50.
Transit in Amsterdam is fairly cheap as well. A strippenkarte (multi-tiered tram ticket) won't cost you much, and most of Amsterdam proper is highly walkable anyway.
Tours in either city will cost you around €15-20, but you don't really need those anyway if you research a bit before you go and want to hit the typical sights.
An evening spent at a weed bar - with 1 gram of good stuff - can cost you anywhere from €15-35, depending on how touristy it is.
Typical coffees or beers at cafes in either city will range from €2-€6, depending on how touristy an area you're in.
Meals can range from €6-€15 (lunch-dinner) if you keep it frugal, double that if you go with run-of-the-mill restaurants.
Most tourist attractions (museums, monuments, etc.) are around the €5-10 range.
The great thing about both cities, though, is that you don't have to spend any of the above money (except maybe food & drink) - simply get to one part of town, walk around, get lost, and soak it all in!