"We hebben nog geen prijs afgesproken."
Translation:We have not agreed on a price yet.
For me, it is equally valid to say "We have not agreed a price yet" - to agree a price is the same as to agree on a price.
Came here to raise this exact same point, and see two people are ahead of me. I don't know if it's a difference between US and UK English, but certainly, in the latter, you can agree something without having to agree ON it - though either would be OK.