I agree, a fair assignment fee is 10-15%, so on $500k, fine, but on a $25k deal, might be a little crazy.
As for the followup question, keep someone from seeing it, (typical lawyer answer) all depends. If you are doing an assignment, buyer has to see it as they are buying the assignment. But if you really want to keep quiet (and it's a cash deal) close it twice. once with your own money (ways around that, but disclosures are involved) and then hour or two later, sell to other person. It will be on public record, but most people do not see it.
Lastly, there is one sure fire way to keep it all hidden, but you need a good lawyer :) Have the seller put the property into a newly formed LLC. The newly formed LLC will have a slightly elaborate operating agreement to allow for all to follow. But you buy the LLC from him and then end buyer buys from you. no public documents involved except transfer to LLC which was originally owned by seller so it's even excempt from Transfer/Conveyance tax :) (which is what you use to pay your attorney) This way has other benefits in that it CAN avoid point of sale with cities and get rid of required escrow deposits on others. BE WARNED, if this is not done right, you can really screw yourself up. You really need to make sure you understand this.
Now, why do I feel like I gave a loaded gun to a kid? If you are going to use this, email me, I'll help where I can.
Good luck :)