句子結構 : 兩個verb

In the sentence:

"that price will go up" is a noun clause which acts as the subject of the sentence.

"will" is the auxiliary verb, "go" is the main verb of the clause. (clause is also a sentence, when it forms part of a longer sentence, it becomes a clause).

"is" is the linking verb, "certain" is the subject complement.

subject complement can be a "adjective", it describes the subject, e.g. The boy is tall, or
a noun, it renames the subject, e.g. The by is a student.

That prices will go up is certain.= the underlined words are a clause, which is subject of this sentence and the main verb is IS.

This sentence can be splitted into two sentences.
That is certain.
Prices will go up.

that ?　 has not returned is strange = same as above.IS is the main verb in this sentence.

This sentence can also be splitted into two sentences.
That is strange.
? has not returned.

It is just another way to say:

"It is certain that prices will go up"

If you find the above acceptable, then the original sentence is also acceptable.