300k-500k normally, assuming you mean just a regular, plain dragon egg. Don't expect them to sell for as low as 300k, it's usually more than that. Plus, they are pretty rare, so once a person sets a price on their egg, it's not likely that they will be open to bargain.
A month or so after I joined empire someone was leaving and offered to sell a dragon egg for 9k, thinking it was a rip-off, I did not buy it..
I actually found it in the frontier. It was in a small dirt hut and the signs sayed "Your welcome *****" It had a dragon egg and a saltar XD