That consultant cost us $2.6 million dollars. Is it cost or costs?

The letter cost $1,800. Is it cost or costs

Please advise, is there a plural for cost or cost can be a noun as well as a verb?
Hello Guest

If you're talking about how much a consultant charged you in the past, you say:

1. That consultant cost us $2.6 million. [no 'dollars']

If you're talking about how much a consultant charges you at present, you say e.g.

2. That consultant costs us $2.6m per annum.

(Presumably he's an IT consultant.)

The same for your letter:

3. The letter cost $1,800 – you sent it last week.

4. The letter costs $1,800 – you're sending it now.

(That's one hell of a heavy letter, by the way.)

And yes, cost can be a noun as well as a verb:

5. The cost of living has gone up by 25% this year. (singular)

6. The defendant was awarded full costs. (plural)

present tense singular: The book costs $10.
present tense plural: The books cost $10 each.

Past tense singular: Last week, that same book only cost $5.
Past tense plural: Last week, those same books only cost $5 each.

This is confusing because all of the past tense forms look just like the third person plural form of the present tense.

Someone more expert in explaining grammar (like MrP) can explain it better than I can. I'm just bored and there aren't any new posts, so I thought I'd contibute my two cents' worth to this thread.
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Your suggestions are very helpful. Please can you give some more examples of plural verb costs. The letter(subject) cost or costs(verb) $15. If we give cost, does the verb agree with the subject. Please advise.
 khoff's reply was promoted to an answer.
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