Promegax's comments make sense. The modifying clause – "which experts termed the most significant event in cultural relics preservations since communist China was founded in 1949" – does seem to refer to the whole action: the paying of a huge sum of money by the Shanghai Museum for something valuable.
For an adjective clause that modifies the entire sentence, "which" is preceded by a comma, as in:
All the students scored very high in the university entrance examinations, which pleased their teacher very much.
I sent out 100 resumes, but got 50 rejection letters, which disappointed me terribly.
The company went out of business, which caused a lot of unemployment in the town.
If the modifying clause referred only to the calligraphic collection, however, the sentence could be written like this:
The Shanghai Museum has paid $4.5 million to retrieve a calligraphic collection WHICH EXPERTS TERMED THE MOST VALUABLE EVER FOUND SINCE COMMUNIST CHINA WAS FOUNDED IN 1949.
Depending on the context, there might or might not be a comma. For an essential clause – one in which the information is necessary to denote what you are talking about – use no commas. In fact, you could use "that" instead of "which":
In a conversation:
A: What's happening? Why is everyone so excited?
B: Haven't you heard? The Shanghai Museum has paid $4.5 million to retrieve a calligraphic collection THAT/ WHICH EXPERTS TERMED THE MOST VALUABLE SINCE COMMUNIST CHINA WAS FOUNDED IN 1949.
or, in another conversation, in which the information in the clause is "extra" (the clause is a non-essential clause):
A: What about this calligraphic collection? Is it special?
B: Yes, it is. The Shanghai Museum has paid $4.5 million to retrieve THIS CALLIGRAPHIC COLLECTION, (comma), WHICH (not "that") EXPERTS TERMED THE MOST VALUABLE SINCE COMMUNIST CHINA WAS FOUNDED IN 1949.
Your sentence, as is, should have a comma before "which" since "which" modifies the entire clause.