I have never seen any expert billing his expertise ("Sachkunde"). It’s more probable that "Sachk." means "Sachkosten".

Sachkosten pl (ReW) costs of materials
(Schäfer, Wirtschaftswörterbuch)

expenditure on material die Sachkosten Pl., kein Sg.
nonpersonnel-costs / non-personnel-costs die Sachkosten Pl., kein Sg.
material costs [FINAN.] die Sachkosten Pl., kein Sg.
cost of materials [FINAN.] die Sachkosten Pl., kein Sg.
https://dict.leo.org/englisch-deutsch/Sachkosten

Costs of materials include direct raw materials, as well as supplies and indirect materials. Where non-incidental amounts of supplies are maintained, the taxpayer must keep inventories of the supplies for income tax purposes, charging them to expense or cost of goods sold as used rather than as purchased.
https://en.wikipedia.org/wiki/Cost_of_goods_sold

The price reflects the final invoice amount based upon the costs of materials, energy and labor in effect at the time the order is placed.
hectronic.com
Der Preis ist bezogen auf den offenen Rechnungsendbetrag und beruht auf den zur Zeit der Bestellung gültigen Material-, Energie- und Lohnkosten.
hectronic.com

The goods receipt or invoice receipt posts the costs of the externally procured materials directly to the sales order item.
help.sap.com
Die Kosten für die fremdbeschafften Materialien werden bei Wareneingang oder bei Rechnungseingang direkt auf die Kundenauftragsposition gebucht.
https://www.linguee.com/english-german/search?source=auto&qu...

4.2.2 Costs of materials
This cost category includes costs for project-related consumables, low value assets, inventory withdrawals and proportional licence fees. Inventory withdrawals must be assessed and accounted for using a legally recognised inventory accounting method (e.g. LIFO, FIFO, moving average).
https://www.ffg.at/sites/default/files/downloads/page/kosten...

(4) Payment for materials is subject to the Allowable Cost and Payment clause of this contract. The Contracting Officer will determine allowable costs of materials in accordance with subpart 31.2 of the Federal Acquisition Regulation (FAR) in effect on the date of this contract.
https://www.acquisition.gov/content/52232-7-payments-under-t...

The costs of materials received would normally take the following into consideration:
(a) Invoice price: This is the basic price at which materials are charged into store. …
(b) Freight: All costs of bringing the materials to the factory should be added to the invoice price for determining the materials cost. …
(c) Sales tax, exciex dury, etc.: …
(d) Containers: …
(e) Cash discount: …
https://books.google.de/books?id=aarxBQAAQBAJ&pg=PA59&lpg=PA...