so basically what you're saying you want to do is do work for clients who have a high probability of not paying you because they don't even know what they want. This can still be accomplished but it still needs a requirements doc. Let them know that they'll be charged for an initial requirements doc, then you'll create a prototype, then a rewrite of the requirements doc. Also charge these clients a 50% retainer for this part of the project.
Prototypes aren't as useful as you would think. Clients see them and are like "hey that's a nice mock up, now make what I want", then you build the actual application and they say "well where's feature X, Y and Z?!?! when you say it wasn't in the prototype they just say "well I thought it wasn't there because it was just a mock up"
Ideally though, you want to charge them for the requirements doc (and get paid for it) before you start the actual project. You can usually tell the client this and tell them they can use the requirements doc to shop around with other developers so they can get the best deal. Clients are usually happy with this and usually don't bother shopping around.
Prototypes aren't as useful as you would think. Clients see them and are like "hey that's a nice mock up, now make what I want", then you build the actual application and they say "well where's feature X, Y and Z?!?! when you say it wasn't in the prototype they just say "well I thought it wasn't there because it was just a mock up"
Ideally though, you want to charge them for the requirements doc (and get paid for it) before you start the actual project. You can usually tell the client this and tell them they can use the requirements doc to shop around with other developers so they can get the best deal. Clients are usually happy with this and usually don't bother shopping around.