Dear …
We appreciate your recent proposal to supply us with __(name of item)__. After reviewing your proposal in detail, we have decided that we just are unable to commit to such an expensive project at this time.
We found your proposal to be very insightful in your attempt to help us accomplish our long-term goals. It is, however, a bit premature for us commit to such a large financial investment.
Thank you, nonetheless, for your time and consideration.
Sincerely,