Månadsavgiften är hög men samtalskostnaden och startavgiften är låga.

Swedish very often forms compound nouns by writing parts together as one word.

Here the compounds are:

• månad + avgift + en → månadsavgiften
• samtal + kostnad + en → samtalskostnaden
• start + avgift + en → startavgiften

This is very normal in Swedish. Where English often uses several words, Swedish often prefers one compound word.

A few useful details:

• avgift = fee
• kostnad = cost
• månad = month
• samtal = call / conversation
• start = start

So:

• månadsavgift = monthly fee
• samtalskostnad = call cost
• startavgift = start fee / connection fee / initial fee

Notice also the -s- in månadsavgiften and samtalskostnaden. That is a common linking sound in compounds and does not by itself mean possession here.

The -en is the definite article suffix. In Swedish, instead of putting the in front of the noun, you often attach it to the end.

So:

• en månadsavgift = a monthly fee

• månadsavgiften = the monthly fee

• en samtalskostnad = a call cost

• samtalskostnaden = the call cost

• en startavgift = a start fee
• startavgiften = the start fee

This is one of the first big differences from English: Swedish usually puts definiteness on the noun itself.

This is because Swedish adjectives must agree with what they describe.

Here the adjectives are predicate adjectives after är.

• Månadsavgiften är hög
  The subject is singular and common gender, so the adjective is hög.

• samtalskostnaden och startavgiften är låga
  Now the subject is actually two things joined by and:
  samtalskostnaden och startavgiften
  That makes the subject plural, so the adjective becomes låga.

So the contrast is:

• singular → hög
• plural → låga

A useful comparison:

• Avgiften är låg. = The fee is low.
• Avgifterna är låga. = The fees are low.

Each noun is singular by itself, but together they form a compound subject:

• samtalskostnaden och startavgiften

In English, this works the same way in meaning:

• The call cost and the start fee are low

Even though each item is singular, the pair is plural together. Swedish therefore uses the plural adjective låga.

So the agreement is based on the whole subject, not on each noun separately.

No. Swedish verbs do not change according to person or number the way verbs do in some other languages.

So:

• avgiften är hög = the fee is high
• avgifterna är höga = the fees are high
• jag är = I am
• de är = they are

The verb är stays the same.
It is the adjective, not the verb, that changes here.

Because Swedish usually marks definiteness with a suffix on the noun instead.

So English:

• the monthly fee
• the call cost
• the start fee

becomes Swedish:

• månadsavgiften
• samtalskostnaden
• startavgiften

The the idea is already included in -en.

Men means but.

It connects two main clauses:

• Månadsavgiften är hög
• men samtalskostnaden och startavgiften är låga

So the sentence contrasts two ideas:

• one thing is high
• the other things are low

It functions very much like English but.

Because after men, Swedish normally keeps regular main clause word order.

The normal order is:

• subject + verb + complement

So:

• samtalskostnaden och startavgiften = subject
• är = verb
• låga = complement

That gives:

• men samtalskostnaden och startavgiften är låga

This is different from some other conjunctions or adverbs that can trigger different word order patterns, but men does not cause inversion here.

They are related, but not identical.

• avgift usually means a fee, charge, or set amount you pay
• kostnad usually means cost or expense more generally

So in this sentence:

• månadsavgift suggests a regular monthly fee
• startavgift suggests a setup fee, connection fee, or starting charge
• samtalskostnad refers more generally to the cost of calls

In many contexts they can be close in meaning, but Swedish often chooses one or the other depending on the type of payment being described.

These nouns are common gender nouns, which is why the indefinite form would use en:

• en månadsavgift
• en samtalskostnad
• en startavgift

That is also why the singular predicate adjective is the common-gender form:

• hög
• låg

If the noun were neuter, you would expect forms like:

• högt
• lågt

But here we do not get that, because the nouns are common gender.

No, not in this sentence.

These are predicate adjectives after är, and predicate adjectives in Swedish mainly agree with:

• gender in the singular
• number in the plural

So:

• singular common gender → hög, låg
• singular neuter → högt, lågt
• plural → höga, låga

Even though the nouns are definite, the adjective here is not using a special definite form just because of that. The important thing is that it is a predicate adjective, not an adjective directly before the noun.

Compare:

• Den höga månadsavgiften = The high monthly fee
  Here the adjective is directly before the noun, so definite marking matters.

But in your sentence:

• Månadsavgiften är hög
  Here hög is after är, so it behaves as a predicate adjective.

Yes. A comma before men is often acceptable and can make the sentence easier to read, especially in a longer sentence.

So both are possible in practice:

• Månadsavgiften är hög men samtalskostnaden och startavgiften är låga.
• Månadsavgiften är hög, men samtalskostnaden och startavgiften är låga.

The meaning does not change. The comma is mainly a punctuation choice.

The indefinite version would be:

• En månadsavgift är hög men en samtalskostnad och en startavgift är låga.

But that sounds less natural in many contexts, because it suggests talking about any monthly fee, any call cost, and any start fee in a very general way.

The original definite sentence is more natural if you are discussing a specific plan, subscription, or pricing structure:

• Månadsavgiften är hög men samtalskostnaden och startavgiften är låga.

So the definite form is probably used because the speaker means the monthly fee / the call cost / the start fee of a particular service.