Krik actually on Ebay the prices are about the same for some reason (I just checked online of the two model types I couldn't even find a 520 but I did find an 871 at $15). But your correct if properly priced 8xx series should cost more then the 5xx. To be honest if you want the best deal you should shop them both because in the second hand market the prices get mixed around quite a bit. I got a 2600 series router for $11 with shipping and a 2500 series router for $22 without shipping. Is your intent to buy new or used leeand00?
The main advantage with new systems is you get a support contract and everything is generally legal. Also there are quite a few discontinued options that Cisco no longer supports.
The main disadvantage to second hand systems no support and cisco doesn't give away IOS. Also it's a bit of a gamble.