I've been mining on NiceHash for quite a while now, and while I love the simplicity of the service, and the payouts have been good. However I have a question about how the btc/ghs/day price is determined.

From what I understand, the site takes an average of all existing, not-dead orders to determine what the current payout is for shares coming in (that number next to each algo on the main page). The problem I'm seeing is when people add orders that are so low-priced, orders which will never actually get filled or worked on, are still bringing down the average payout price. Could this be abused by simply adding a bunch of lowball orders that never will receive any hash power, just to lower the overall average? Or are those orders marked dead after sitting idle for x time? If not, wouldn't it make more sense to take the average of the orders that are currently being worked on by miners, as opposed to including ones sitting there doing nothing?

Sure, this could be offset by using the price limit feature, but they seem to have encouraged a lot of people to not use it. If more miners used the price feature, people putting in very low orders would be guaranteed to never receive hash power, even if the service get DDOS attacked, and new orders cant be put in.