Or "net amount", see below.

After several related the questions there is no longer much doubt about the identity of the source document, which is assumed below.

The court explains, two sentences further down, what they mean by "prestation":

"dvs. varje insats och vinstutbetalning", in other words, every bet and payout.

One of the parties placed bets, the other party made payouts.
One thing these bets and payouts have in common is a monetary value.

This makes it possible to calculate the net amount that one party needs to pay the other party, which is the "nettobelopp" mentioned.

Probably no direct equivalent in English, since:

"anything you did under the void contract is generally not recoverable, and you can't seek restitution."

Source: https://www.investopedia.com/terms/v/void-contract.asp


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Note added at 1 day 1 hr (2024-01-26 01:53:31 GMT)
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The litigation is about "belopp" (i.e. amount, a word that was used 53 times).
The payments are just one component of the disputed amount.
Other components include: interest on payments received and cost of litigation.
And possibly more.

The court was careful to include an important "m.m." (i.e. etc.):
"dvs. varje insats och vinstutbetalning m.m."

"... P.L. ska därför tillerkännas ersättning för hälften av sina rättegångskostnader.. ."

"... BML Groups avkastning på P.L. insatta kapital. Avkastningen utgör obehörig vinst. Beloppen avser återbäring av den obehöriga förmögenhetsöverföring som skett till följd av avtal som är ogiltiga"