The term "Coûts maturés" seems somewhat atypical. I have never seen it in a French construction contract or heard it used when negotiating with subcontractors. In my opinion, this is a standard case of "estimated/projected costs" (given at the time of le devis, le travail prévu) and "actual cost" (cost after completion of the work, travail effectué). Let us not forget that milestone payments may also come into play here, in which case the "actual cost" may contractually evolve depending on the circumstances.

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Note added at 1 heure (2013-01-23 12:02:10 GMT)
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If milestone payments (or progress payments) do figure into this document, then "les coûts maturés" may account for any authorized or unexpected increases.

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Note added at 1 heure (2013-01-23 12:09:11 GMT)
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In this case, the phrase "coûts maturés" may refer to "la valeur acquise," or "completed construction cost."

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Note added at 1 heure (2013-01-23 12:10:50 GMT)
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cost(s)

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Note added at 3 heures (2013-01-23 14:19:28 GMT)
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"Costs of construction actually incurred"

I am guessing here because it's a term I have never seen in French and can't imagine any justification for, but I'm wondering whether 'maturé' could be a (somewhat ill-advised) reference to maturity in the sense of 'the final payment date has arrived'.

See:
http://en.wikipedia.org/wiki/Maturity_(finance)
In finance, maturity or maturity date refers to the final payment date of a loan or other financial instrument, at which point the principal (and all remaining interest) is due to be paid.

http://fr.wikipedia.org/wiki/Maturité_(finance)
La maturité ou maturité résiduelle (en finance) désigne le temps qui sépare la date à laquelle une obligation est émise, et la date à laquelle la valeur nominale de cette obligation est remboursée. Par contre, le terme maturité est un anglicisme. Le bon terme à utiliser est échéance.

Actual costs keep growing until they are "final", in a sense that they won't grow any more. Like someone who finished growing, arrived at "la maturité".

Imho