Русско-англо-русский юридический словарь

Once you agree on a price, make sure to split the payments in such a way that the final payment (a substantial portion of the total amount) will be made only when the work has been fully completed and you have signed off on it. (Forbes)

In the US, we usually say “agree on a price.” I’m also comfortable with “agree upon a price,” although I suspect the word “upon” may be falling out of use in all contexts. I can’t tell you that no one says “agree a price,” particularly in the UK. (Quora)