Probably relevant to mention that "Gengo" is the Japanese word for "language". So it makes a lot of sense for us.
It was owned by someone who's name is actually Gengo, and had a business predicated on the name. So he was reluctant to sell.
The broker got between 10-15%.
(I've been doing this since 1996...)
Regardless, hard to have a perfect transaction in this kind of thing from a buyer's point of view because by default you have imperfect information. Such is life.
I've done something similar. Base rate plus anything under "x" I get a percentage of the "savings". But for me it's less of an issue as I do this mainly for fun. I did it for free in the beginning but it ended up taking to much time so I started to charge to slow things down. I just had a deal where I bought a domain for $3000 less after my buyer already had agreed to the sellers price and the deal was done. I smelled blood in the water so I got the seller (w/o the buyers knowledge) to lower the price more just for the fun of it.
In general I will see what the budget is for the purchase and if not reasonable won't take the assignment at all.