firm price = čvrsta cena
Nada Vukčević - Finansijsko-bankarski rečnik
Milica Gačić - Rječnik prava

firm price – A designation that a quotation (a bid or offer price) will not be changed for a specified period of time (i.e., “firm”)
http://www.msrb.org/msrb1/glossary/view_def.asp?param=FIRM

firm price contract – Alternative term for fixed price contract
http://www.businessdictionary.com/definition/fixed-price-con...

fixed price contract – Contract that provides for a price which normally is not subject to any adjustment unless certain provisions (such as contract change, economic pricing, or defective pricing) are included in the agreement.
http://www.businessdictionary.com/definition/firm-price-cont...