Here are two clear ways to rephrase your message so it sounds polite and easy to understand. Each version is followed by a short note about why it works.
-
We don't have an active project right now, but could you give a rough estimate for this sample brief so we can see if your price fits our budget?
Why this works: Short and friendly. It explains there is no current project and asks for a rough estimate to check the budget.
-
Although we aren't working on the project at the moment, would you be able to provide a ballpark figure for the scope described in this sample brief? That will help us decide if your quote is within our range.
Why this works: Slightly more formal but clear. It asks for a ballpark cost and explains the purpose — to check whether the quote is suitable.
If you want the tone to be more casual or more formal, tell me and I can rewrite these to match.