Present Continuous doesn't really work here. For general properties of an object or a person (e.g. temperature, size, price etc.), you should use Present Simple:
The stove feels hot. The painting costs a lot. The box has the shape of a cube. The dog stinks; it needs a bath.
I think this is a great question. For example, if you look at the Dutch Grammar page at http://www.dutchgrammar.com/en/?n=WordOrder.02 this indicates that the prefix of the phrasal verb should proceed only the infinitive verbs.
In this sentence, however, there are multiple finite, intranstive constructions (i wonder & it costs). Anyone have the word order rules for sentences like these?
As a native English speaker :), I would say your suggestion is in good English, but you have split the translation into two main clauses. It would not be wrong, therefore, to write "I wonder! How much does it cost?" or better, use a semicolon, as in "I wonder; how much does it cost?" However, semicolons are mainly used in formal writing. Most people would use a dash, as in "I wonder - how much does it cost?"
I'm definitely not an expert in Dutch, but the given translation is in very good English.
I hope you find this useful - if a little pedantic!