Unless you are working on a profit-share type deal where you are going to do the work at a cut-rate price and share in the profits later, most clients in my experience want to know what something is going to cost as early as possible (even it it's just a guesstimate).

You can work as much as you like on a proposal and it can, on paper, be the greatest thing ever and promise to increase profits by whatever-percentage but if it's something that is going to be out of reach of the immediate budget of the client, then you're wasting your own (and their) time.

I have been involved in multi-million £ proposals and when it got to the end it turns out the client was expecting something for 50k. Would have been much easier to lay the cards on the table earlier in the process.