"Cost" is a noun, of which the plural is "costs". "Cost" is also a verb, of which the thrid person present indicative is "costs".

The cost of this advice is £5. Sorry, but that's how much it costs! If you can't bear such costs, don't ask questions.

Na ja, das eine ist Singular, das andere Plural … Normalerweise immer "cost", aber wenn es sich um mehrere Beträge handelt, dann costs.

Also "The cost of implementing module ABC is 200,000 euros." Aber "The costs of implementing the three modules are 600,000 euros."

Eigentlich ganz simpel. Bei "Kosten" gibt es keinen Singular, daher haben viele Schwierigkeiten mit "cost". Denk einfach an "Betrag" statt "Kosten", dann wirst du's einigermaßen richtig machen.

Ups, da sollte noch ein "in total" dahinter.

Sorry, Sophil, but "The costs of implementing the three modules are 600,000 euros" looks wrong to me. I would say "The cost of implementing all three modules is 600,000 euros."

The essence of this is that it is the same kind of cost. I admit that this is getting difficult, but maybe another example might help:

If we are considering implementing all three modules, we must take into account the following costs: personnel, materials, financing etc. All of these individual costs must be taken into account when estimating the total cost.

Definition - Cambridge Advanced Learner's Dictionary

cost (MONEY)
noun [U]
<the amount of money needed to buy, do or make something:>
//
When you buy a new computer, you usually get software included at no extra cost (= for no additional money).
For many parents, two salaries are essential to cover the cost of (= pay for) school fees.
The supermarket chain announced that it was cutting the cost (= reducing the price) of all its fresh and frozen meat.
It's difficult for most people to cope with the rising cost of (= increasing price of) healthcare.
I was able to buy the damaged goods at cost (= for only the amount of money needed to produce or get the goods, without any extra money added for profit).
//

costs
plural noun
<the cost of something:>
//
We need to cut our advertising costs.
The estimated costs of the building project are well over £1 million.
//

Joe W, auch mit dem "in total"? Das ist mir beim Editieren irgendwie rausgerutscht, wie ich oben bereits schrieb.

@Joe W

Kann ich auch in $ bezahlen? ;-)

Vielen Dank an alle!!! Ihr habt mir sehr geholfen!!!

@ sophil

even with the "in total" in that sentence, I agree with Joe that it looks wrong. You have the amount of 600,000 for implementing the three modules. That is the cost of implementing it. However, if it were unspecified, you might say "the costs involved in implementing the modules are enormous"

This reminds me a bit of "people" vs. "peoples". If I say "the peoples of the world", I'm emphasizing the variety of cultures. Mathematically speaking, you can have as many people as you like and it's still just people. Similarly (well, somewhat similarly -- this isn't a perfect analogy), if I say "costs" I'm emphasizing the fact that the costs are coming from different sources, or at different times, or perhaps are not all monetary. It's also a more vague notion; a definite figure like $600,000 is a cost, but if I don't have a definite total *and* the cost has more than one source (say, buying equipment *and* hiring people), then I'd say "costs".

  related discussion:Kosten --> cost or costs?

Hmmm, ihr habt Recht -- ich habe meiner eigenen Regel widersprochen: "Die Beträge der Realisierung der drei Module sind 600.000 Euro." klingt auch falsch. :-)
War also wohl ein schlechtes Beispiel. Wie wär's mit "The implementation costs of the three modules added up to 600.000 euros."?

Joe, sure, I had already noticed the punctuation flaw, but too late.