Hey Danny,
GravityForms with the WooThemes Gravity Forms addon will allow you to create forms for a product. What I do, is I have a form for a custom request. Then the vendors create a Custom Request listing, and I assign it that form on the edit product page. When a buyer goes to use the form, they can input the price, input what they are paying for, and checkout! Easy as that. 🙂 It’s like having a “Custom product just for you” page every time. Very handy!
And sure, vendors can set the prices and change them at any time. 🙂
Cheers
Ben