Rolling three posts into one ball o'wax:

I'm talking with the client this afternoon ... I am asked, without full knowledge of the issues, for an estimate of the cost to do a number of infringement opinions.  I do so, and emphasize that it is, indeed, just an estimate, and based upon incomplete knowledge of the scope of the effort.

I next send along an engagement letter and ask for a retainer to cover slightly more than half of the estimated costs.  Client writes back and says your approach looks sound, go ahead.

At which point, I write back and say, "I appreciate that time is of the essence.  I anticipate X hours of review, which is necessary to fully understand the scope and details of your matter before I even stick my foot into this tar baby (see Uncle Remus, children).  I'm ready to proceed.  Send the retainer as detailed in paragraph Whatever, along with the signed service agreement.  If/when the potential not-yet-a client says, 'gee, that sounds like a lot," I explain . . .

he's welcome to do that and then I pause. We agree that I will revisit my original estimate as I now know more about the patents at issue and we will agree on a retainer.  (Not sure I've ever had a client try to negotiate a retainer before, either.)

So, I'll now re-estimate the work involved and get back to him, but there's something about this deal that makes me uncomfortable.

Hold it!

Are you saying to the client, "Well, I'm not very experienced, at least not enough to evaluate the breadth, depth, and cost of the cases I take.  So, yeah, help me out here."  That you can't judge the magnitude of the cases you take?

I didn't think so.

I would consider my estimate of the work based on the no-fee review I did before the not-a-client balked at the price.

In my family law practice, aspects of the case can include dissolution of marriage whether hotly-contested or agreed-necessary, hot-or-not parenting issues, the molten-lava contested issue of homemaker receiving more than 50% of the marital estate for rehabilitative maintenance purposes, etc., and how much gratuitous crap I will have to deal with as a professional byproduct of (1) practicing family law, (2) with this particular person, (3) etc.
And then I present the estimate-only figure, the retainer that remains in the trust account toward the last bill, my client information sheet, and the service agreement.  Sometimes I do flat-fee stages, sometimes I do it hourly.  If I perceive a troublesome client but I have time, I'll bill hourly.

What I learned to do:  Write down various client questions and objections and write down your cleanest shortest answers.  Practice them when the phone rings (nobody can see you pull out your cheat sheets). 

Picture your old raggedy-ass self sitting on a cold stoop somewhere, sucking cheap red from a bottle in a paper bag, and thinking, "damn, I shoulda I coulda I'd do that now, damn."

I hesitate to say flat-out that "I don't brook no nonsense."  Hell, I practice family law.  Mostly all involved deal with nonsense at some point in a case.  And a client can always always ask clarifying questions and "why that way" questions/ But the client HIRED ME to do the work. I do not deal well with challenges.  Definitely ask a question, but don't confront me with how much better you would do it. 

That's deadly.  Or what "my mom says".  Frankly, grow a set and and call me back.