Der Arbeitspreis wurde zweimal abgerechnet.

Because this sentence is in the passive voice.

German forms the dynamic passive with:

werden + past participle

So:

• wurde = past tense of werden
• abgerechnet = past participle of abrechnen

Together, wurde abgerechnet means was billed / was charged / was invoiced.

If you used hat abgerechnet, that would be active voice and would need a subject that does the action, for example:

• Der Anbieter hat den Arbeitspreis zweimal abgerechnet.
  The provider billed the work/usage charge twice.

In your sentence, the focus is on the thing affected by the action, not on who did it.

In a passive sentence, the thing receiving the action often becomes the grammatical subject, so it appears in the nominative.

Compare:

• Active: Der Anbieter rechnete den Arbeitspreis zweimal ab.
  Here, den Arbeitspreis is accusative.

• Passive: Der Arbeitspreis wurde zweimal abgerechnet.
  Here, der Arbeitspreis is now the subject, so it is nominative.

That is why you see der, not den.

Because German puts the past participle at the end in this kind of passive clause.

The structure is:

Subject + finite verb + other elements + past participle

So here:

• Der Arbeitspreis = subject
• wurde = finite verb
• zweimal = adverb
• abgerechnet = past participle at the end

This final position is very common in German with:

• perfect tenses
• passive constructions
• modal verbs plus infinitives
• subordinate clauses

Yes, abrechnen is usually a separable verb.

In a simple active sentence, the prefix separates:

• Wir rechnen den Arbeitspreis ab.

But in forms with a participle or infinitive, it comes back together:

• abzurechnen
• abgerechnet
• wurde abgerechnet

So yes, it matters: that is why the participle is abgerechnet as one word, even though in some other sentence forms you would see rechnet ... ab.

It is a compound noun, which is very common in German.

It is made from:

• Arbeit
• Preis

In technical or utility contexts, Arbeitspreis is a fixed term, often contrasting with something like Grundpreis.

Grammatically, the important point is that the last part of a compound noun determines the gender. Since Preis is masculine (der Preis), the whole compound is also masculine:

• der Arbeitspreis

That is why the sentence begins with Der Arbeitspreis.

Because the subject is singular:

• der Arbeitspreis = singular

So the verb must also be singular:

• Der Arbeitspreis wurde ...

If the subject were plural, you would use wurden:

• Die Arbeitspreise wurden zweimal abgerechnet.

Zweimal is an adverb meaning twice.

Here it tells you how many times the billing happened.

Its position in this sentence is very natural:

• Der Arbeitspreis wurde zweimal abgerechnet.

It usually comes in the middle field, after the finite verb and before the participle.

Other placements are sometimes possible for emphasis, but this is the standard, neutral order.

Yes, but that would be a different tense.

• wurde abgerechnet = simple past passive
• ist abgerechnet worden = present perfect passive

Both can often translate similarly into English, depending on context, but German uses them differently.

Your sentence:

• Der Arbeitspreis wurde zweimal abgerechnet.

is a normal written-style way to report a completed past event.

A version with ist ... worden would be:

• Der Arbeitspreis ist zweimal abgerechnet worden.

That is also correct, but it sounds more like spoken German in many contexts.

They are related but not identical.

• berechnen often means to calculate or to charge
• abrechnen often means to bill, to invoice, or to settle/account for something

In many real-life contexts, especially billing, abrechnen suggests that the amount was actually processed or put on the bill, not just mathematically calculated.

So in this sentence, abgerechnet fits the idea of something being billed twice.

Yes. You just need to add the person or thing doing the billing.

For example:

• Der Anbieter rechnete den Arbeitspreis zweimal ab.
• Die Firma hat den Arbeitspreis zweimal abgerechnet.

Notice the changes:

• the doer becomes the subject
• der Arbeitspreis changes from nominative to den Arbeitspreis in the accusative
• the verb becomes active instead of passive

Yes. This sentence is a very good model for a common German pattern:

Subject + werden (past) + adverb + past participle

For example:

• Die Rechnung wurde gestern bezahlt.
• Der Betrag wurde falsch berechnet.
• Die Gebühr wurde zweimal erhoben.

So Der Arbeitspreis wurde zweimal abgerechnet is not unusual at all. It follows a very standard German passive pattern.