How do you get eggs anyway? I've never even attempted it, I don't even know who uses them...
I think the reason for the high value on them though is that a lot of the more powerful characters who need them in large quantities (A box being 35 at a time) probably have more interesting things to do with their time than mess around collecting eggs, either that or because it's a tedious job they don't mind paying other characters to do it on their behalf. Yes, 5000 is a lot for a box of eggs when you think about it logically, but its all to do with supply and demand, if the characters that can afford to pay that much had the time or inclination to do it themselves the prices wouldn't be so high.
That's my take on it anyway. I'd probably sell them myself for a quick coin when it was convenient for me to do so, as my weaponcrafting isn't making me much gold at the moment
