Make sure you visit the "Real" statue, it's one of those life list things, and a must do.
The museum is only a couple of Euro to get in.
There is not much else to see but the Statue, a few other statues in a side area but your there for the David.
The staff will prevent you from taking photos, which is highly annoying.
However if you want a Facebook moment photo, there is a full size copy in the square in the city centre, commonly known as the "Fake David" by the locals I talked to.
The gift shop was very reasonable prices for some items so you can still get a something to remember the visit. I got a small David Head (~15 cm high) for about 20 Euro. less than 1/2 I was expecting to pay.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.