Die Grundgebühr ist teuer, wohingegen die Prepaidkarte günstig ist.

Wohingegen is a subordinating conjunction that introduces a contrast, similar to whereas, while, or by contrast in English.

In this sentence, it links two ideas:

• Die Grundgebühr ist teuer
• die Prepaidkarte günstig ist

and shows that they are being compared in opposition.

It is a bit more formal and explicit than just using aber.

Because wohingegen introduces a subordinate clause, and in German subordinate clauses are separated from the main clause with a comma.

So the structure is:

• main clause: Die Grundgebühr ist teuer
• subordinate clause: wohingegen die Prepaidkarte günstig ist

That comma is required in standard German.

Because wohingegen is a subordinating conjunction, and subordinating conjunctions send the conjugated verb to the end of the clause.

Compare:

• Main clause: Die Prepaidkarte ist günstig.
• After wohingegen: ..., wohingegen die Prepaidkarte günstig ist.

This is one of the most important German word-order patterns to learn.

Yes. Both Grundgebühr and Prepaidkarte are feminine singular nouns, so they take die in the nominative singular.

A helpful rule with compound nouns is that the last part determines the gender:

• die Gebühr → die Grundgebühr
• die Karte → die Prepaidkarte

So even if the first part changes, the final noun controls the gender.

Both are in the nominative case because each one is the subject of its clause.

• Die Grundgebühr ist teuer.
• die Prepaidkarte günstig ist.

The verb sein often links a subject to an adjective, and the subject stays in the nominative.

Because here they are predicate adjectives, not adjectives placed before a noun.

After verbs like sein, werden, and bleiben, German adjectives usually do not take endings:

• Die Grundgebühr ist teuer.
• Die Prepaidkarte ist günstig.

But before a noun, they do take endings:

• die teure Grundgebühr
• die günstige Prepaidkarte

So:

• ist teuer = no adjective ending
• die teure Grundgebühr = adjective ending needed

Yes. A very natural alternative is:

Die Grundgebühr ist teuer, aber die Prepaidkarte ist günstig.

The meaning is similar, but the grammar changes:

• aber is a coordinating conjunction
• wohingegen is a subordinating conjunction

That means with aber, the verb stays in normal main-clause position:

• ..., aber die Prepaidkarte ist günstig

With wohingegen, the verb goes to the end:

• ..., wohingegen die Prepaidkarte günstig ist

In tone, wohingegen often sounds a bit more formal or written.

Usually no, not in this exact sentence.

Sondern is used after a negated statement to correct it:

• Die Grundgebühr ist nicht teuer, sondern günstig.

or

• Die Grundgebühr ist nicht teuer, sondern die Prepaidkarte ist teuer.

In your sentence, there is no negation. It is simply a contrast between two true statements, so wohingegen or aber fits better.

Yes. German very often combines words into a single compound noun, so Prepaidkarte as one word is normal.

You may also sometimes see Prepaid-Karte with a hyphen, especially in advertising or for readability, but the one-word form is completely standard.

Also remember:

• all nouns are capitalized in German
• so Prepaidkarte must begin with a capital letter

Yes. You can switch them:

Die Prepaidkarte ist günstig, wohingegen die Grundgebühr teuer ist.

That keeps the same basic meaning but changes the emphasis slightly.

You can also start with the wohingegen clause, but that is less common and can sound more formal or literary:

Wohingegen die Prepaidkarte günstig ist, ist die Grundgebühr teuer.

Grammatically it works, but most learners will sound more natural using either:

• Die Grundgebühr ist teuer, wohingegen ...
• or simply ..., aber ...

Yes, it is grammatical and natural, especially in written or semi-formal contexts such as comparing phone plans, contracts, or pricing options.

That said, in everyday speech many native speakers would more often say:

Die Grundgebühr ist teuer, aber die Prepaidkarte ist günstig.

So both are correct, but:

• wohingegen = a bit more formal, more explicitly contrastive
• aber = more common in everyday conversation