Learn Dutch: Niet alleen de prijs is exclusief, maar ook de service. - Not only the price is exclusive, but also the service.

Maar (but) is a coordinating conjunction, just like en (and) or of (or). It simply links two clauses together and sits in 'position zero', meaning it doesn't count towards the word order of the sentence that follows it. Adverbial connectors like bovendien or daarnaast, however, take up the first position and force the verb to come before the subject (V2 inversion).

Is niet alleen ... maar ook a fixed combination?

Yes, it is a paired conjunction that translates exactly to 'not only ... but also'. It is a very common way to add emphasis when connecting two things, serving as a great alternative to using words like bovendien (moreover) or verder (furthermore).

If niet alleen is at the beginning of the sentence, why isn't there verb inversion (Niet alleen is de prijs...)?

In this specific sentence, niet alleen isn't acting as an independent adverb; it directly modifies de prijs. The entire phrase Niet alleen de prijs (Not only the price) acts as one single block in position one. Therefore, the verb is takes its normal place right after it in position two.

Is it grammatically okay to leave the verb out at the end of the sentence like this?