If you know what you need, tell them exactly what you need and if they are experienced in that, they can usually give you a flat fixed fee. If you don't know what you need, ask them what they think you need and what a flat fee would be on it. Even if there are some variables in terms of how much time may be required, they should be able to give you some kind of idea for at least the fixed part and an estimate on the variable side. In this economy, lawyers and law firms know that they need to be flexible with clients and can work deals to discount hourly rates.
However, that being said, don't go with someone just because they are cheaper. You are hiring a lawyer for their expertise and advice. You need to ask them questions so you feel confident that they know exactly what they are doing. Realize that hiring a well known firm's partner at a higher hourly rate may sound expensive, but their experience with your issues and the lower amount of hours needed to complete something due to this experience can be well worth it.