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.
I'm with Janet427540 here. Dutch does not include the preposition "on", and neither does UK English. I recall reporting this a little while ago when I passed this way but it seems that D/L does not share our opinion leading me to believe it may be a requirement of US English.
But it's not needed in English. The "on" is superfluous in English Maybe it's required in American?