Concerning clauses - Is this a grammatically correct sentence?

Well Beer to me it looks perfectly ok without

I am impressed by your determination to learn English, a very difficult tongue

This is actually,

The company's revenue for 2013 was $x, which was a 25% increase over 2012.

'which was' has been dropped, which is very natural for English.

The company's revenue for 2013 was $x, a 25% increase over 2012.

There's nothing wrong with your sentence. Main parts are:

revenue / was / $X

The clause "a 25% . . . 2012" " modifies (explains) $X.

@ JTT and Punkey

Your answers raised a question: in formal writing, does the clause need to have a reference to revenue?

The company's revenue for 2013 was $x, a 25% increase over that of 2012.

Something like, "The sales of tomatoes increased, but that of bananas fell." Can I say, "The sales of tomatoes increased, but bananas fell?"

'Sales' are plural, so you would need 'those', not 'that'.

It's a bit unclear.

Better, "The sales of tomatoes increased, but fell for bananas."

Also, in your question the question mark goes outside of the parenthesis, otherwise it would seem as if you were quoting a question within your question.

Interesting. I thought sales was singular: sales manager, sales engineer, sales strategy, etc., and the "s" was to distinguish from sale (selling at reduced prices).

In that sense it is used as a the name of a department or section of a business, and is a noncountable noun. Sales of tomatoes or whatever are clearly countable, otherwise how would we know they increased?

Also a sale can be a low-cost selling event but its more usual common meaning is simply a transaction in which something is sold. The seller makes a sale and the buyer makes a purchase.

English is not your first language, I think?

You have changed your original question.

By inserting the word "but" (connecting conjunction) you must create two complete thoughts.

He likes pizza, but I like pasta.

Sales for tomatoes are good, but banana sales fell by 25%.

Even if you leave out the second "sales," it it implied.