Charge for your time.

If they're not willing to shell out a hundred or two for your consultation, they're not your client. Offer to credit upon install. And give them a ballpark on site, while chatting take notes with numbers associated. Show them the notes, here's your budget. Then if they want/need more numbers, charge again for design.

Best Buy will do it for fee, we'll we all know you get what you pay for.