I don't know what the second stage you described entails but do you think it might help if you include in the first email an open ended question, asking the customer to describe what they have in mind? Not just ideas but a verbal picture of what they want. This pushes you customer to not just throw out ideas but bring those ideas into one unit or two. If they have several pictures, they could include what aspects of each cake they want in the final product. Think of instances where you've requested an insurance quote online, those questionnaires stop short of asking what color your underwear is. What I'm getting at is: get as much info as you possibly can during your first contact with the customer.
Also, there is a new thread that shows how Paul Bradford deals with initial requests for information from potential customers. His ideas are really good.
I don't think you should be spending an inordinate amount of time coming up with a quote, especially since the sale isn't guaranteed at that point.
If pricing is your issue, you might want to do what some here have done: set up your pricing in tiers e.g. level 1, 2, etc. Give enough wiggle room in each level to accommodate a reasonable amount of decorations or extras so that when a customer describes what they want, you both can settle on a price level, and you both have wiggle room to modify designs without you 'nickeling and diming.' Once they are committed, you can go into details etc. At this point, they are paying for your time.