What does “schon etwas” mean and when do you use it? I could not find it on any vocabulary!
I wouldn’t consider “schon etwas” to be a fixed expression. Here “etwas” has the meaning of “somewhat”:
Es ist etwas teuer = It’s somewhat expensive
“Schon” (besides its use as an adverb meaning “already”) may act, as in this case, as one of the so-called “modal particles” which are a very characteristic feature of the German language.
Wikipedia elaborates on this topic and explains the use of the main ones, including “schon”:
“schon etwas” ==> “somewhat” or "a “little bit”
You use it in German if you carefully want to indicate that something is not OK,
for example: “das ist schon etwas traurig”, “das ist schon etwas ungerecht” …
Just to add to the confusion:
When ‘etwas or ziemlich or recht’ [rather, quite] have ‘schon’ in front of it, things get really interesting.
Schon is one of those words that often is not translated into English (and has a myriad of subtle shades of meaning in German).
In your example it is an expression of a certain reserve or limitation: “Yes, you are right, it is rather expensive (and I might not want to buy it”). Exactly as Hajo_Smith has written below.
Exactly Susanne,
I’ve just discussed that nuance you talk about with “schon recht” because I happened to come across it this morning in a quaint little book I was reading.
Here is the full sentence:
Das ist mir schon recht → that’s all right by me.
Whilst “Das ist mir recht” is absolutely fine, it happens to be more formal.
Apparently, “Das ist mir schon recht” is rendered more colloquial by the use of “schon”.
Moreover I’ve heard that it can even be shortened to “schon recht.”
So depending upon how “schon” is used, it can act as a softener 
Das ist schon wahr 