Past participle and which

john liaomusic prices

What is this? The price of CDs? The price of musical scores?

john liaoto high

too high

john liaothe real thing

What is this? You haven't mentioned anything that would be "the fake thing", so it's hard to imagine what you mean by "the real thing". Real music

The whole context:

There's also the argument that people use file-sharing sites to decide whether they like, say, a particular album and if they do, then they'll go out and buy it legally.
Historically, music prices have simply been to high and that most people would be happy to buy the real thing provided the prices were lower.

john liaothe real thing provided (that) the prices were lower.

My answer to your original question is the same.

john liaomusic prices have simply been to high and that most people would be happy ...

"that" should be omitted. It makes no sense there.

CJ

So, should be "that" rather than "which" here?

john liaoSo, should be "that" rather than "which" here?

Yes.

Use
provided
or
provided that
but not
provided which.

CJ

You mean here the clauses is a defining relative clause, not a non-defining clause?

john liaodefining relative clause

There are no relative clauses in your sentence, neither defining nor non-defining.

CJ