I have to be blunt (all right, when am I not?) and say that asking for a reduced commission rate is not a great way to start a relationship with an agent and is possibly going to mean you have no relationship with the agent concerned.
(The following may sound like a whinge but I've been moved to write it because this sort of thing can lead to misunderstandings and upsets, and we genteel folks in the publishing industry like to avoid those sorts of things.)
Agents, perhaps more than most people in the publishing industry, understand how little money is involved in most advances - we see offers for books; we also look at royalty statements. We get it - authors are not well compensated for their work. We try to get them the best compensation we can, but there's simply not that much money goin' around and there are a lot of people trying to share it. So we can also understand, in principle, why an author may want their agent to take less of a share.
However, the author is asking the agent to do work for them. The usual understanding of work in a capitalist economy is that you get paid to do it, unless it is specifically termed 'volunteer'. Agents are not volunteers. We have to pay rent, electricity etc, and that's just in the workplace. No agents I know of are jetting off to Monte Carlo to play in high-stakes games and hobnob with Their Serene Highnesses. No, no - we are paying our bills and getting by and mostly doing this job because we love books, not because we expect to make huge profits (because we don't).
So when an author asks an agent to drop their commission rate, what they're basically asking is for us to work for less in an already thin-margin environment. How would that author feel if their agent was offered, say, $10 000 for the author's book and said to the publisher, 'No, that's okay - we'll take $8000'. Author probably wouldn't be happy, right? Well, that's how agents feel when authors ask them to take a pay cut.
There is also this: taking on a new author is a big financial risk. Agents can invest a lot of time and skill in a new author and if we don't end up placing the book there is nothing to remunerate us for the time and skill. When we think about taking on a new client we weigh up that risk, but it doesn't take much to tip it into too-risky territory. If we're asked to drop the commission rate the risk often becomes too great, and then it's no longer viable for us to take on that author, with the consequence that we don't take them on.
Of course, the author may be bringing a backlist with them, or they may have a certain amount of bargaining power, and in those circumstances there can be room to move. But with first-time authors there is usually very little room to move on that commission rate. I would love it to be the case that advances are so high that we could take a minute amount of commission and still be able to keep the lights on, but there isn't a country in the world where that is the case - not for books, where the amounts of money in each territory are fairly small relative to other intellectual properties like screenplays or acting. And actors agents don't take 5% commission either.
In summary: the reality is that if you want to have an agent, you're going to have to pay them commission because they are doing work, not volunteering. What you're paying them is the most reasonable rate they can charge you and still make taking on your manuscript viable. If you tell them that you want to pay less than that reasonable rate, don't be surprised if they don't embrace you with open arms, just as you wouldn't like someone trying to pay you less for your professional services. If you think that an agent isn't worth the money then the solution is easy: don't have one.