If these are tire-kickers that have never made a payment to you before, there are three options (all require a payment in advance and no additional effort from you in advance):
A. Mechanical Delivery: with a non-refundable deposit, based on scope they provide and change order policy should you determine their scope is under-developed. If you choose, the deposit could go towards Option B.
B. Project Discovery: you develop scope, discuss alternatives, deliver a project document with options/budget
C. Project Theory: with a non-refundable deposit, (by phone) you complete an interview filling out a form you have developed (okay, there is some advance effort on your part): What is the purpose? What is the source? What is the destination? What data files will you import/export to/from? Etc. The deliverable is the completed form which they can then take and get their start towards defining their own scope. If you choose, the deposit could go towards Option A.
The titles could use some work but I imagine these are priced:
A. $$$
B. $$
C. $
Not because there is more/less work but because of the potential headaches and pitfalls.
I feel like we need to offer C as an option so we can capture some revenue and turn them into a paying customer and so we can establish a point that it doesn't happen without some time, effort, and experience. Either they understand or they never come back (but we've got a couple of bucks in our pockets).