I find that its helpful to ask what a client's budget is and then tell them what you can do for that amount. Typically, at the commercial level, they will come to you with their budget already set, so this manner of negotiation can save yourself from underbidding.
That said, you can also present a range of prices for two different levels and go from there. For example, at my design studio we specialize in poster design for the entertainment industry. When a new client asks about our rates, I will tell them that we have two budget ranges for one-sheets; $15-25,000 for an independent project and $25,000-300,000 for a studio project, then I'll give them particulars of what's involved in terms of copywriting, photography, number of comps, etc. But this rarely happens anymore. In most cases, our fee is determined by what they've allowed in their budget. They'll call us up and say we have X amount in our budget and we need it by so-and-so. For your regulars it's important to stay flexible. Sometime's they don't have the normal budget and will ask that you take on the job with the understanding that you'll make it up on the next one. By giving them a range of choice it makes them feel like they're in control.
