decibel

Decibel (dB) - The measuring unit of sound pressure, and hence loudness. The decibel is a numerical ratio between the sound pressure of a given sound and the sound pressure of a reference sound (usually .0002 microbar).

I've been puzzled by something for a while now. There are questions at the end, but the preamble is my attempt to think my way through some of the background info. There may be plenty of mistakes along the way, so feel free to point them out. That said, here we go:

Is there one that exists for a decent price? And why are there such big differences in price? It's a meter...It's a single function device, so why do some go for $30 and some for $150?

I am confused about the decibel scale that is used on meters. I have been reading and trying to understand how meters work. I came up on a article where it was mentioned that 0db is the 'threshold of hearing' and then later in the same article it was mentioned that digital audio has a limit of 0db.