1 ms will last you 30 days.
if you can't make 500k in 30 days, you're doing something wrong, plus the added benefits of MS pokes, MS hunting areas and the bonus xp and gold make it still worth that price.
There are more pokes for sale, causing more inflation.
More emphasis on PVP pokes causing large sums of money to switch hands often.
There are many factors to the price changes, not the least of which being, if people are buying the MS for 500k, then they will keep being sold for that price. If no one buys them at that price, the sellers will drop the price. If you're upset about the price that much, buy yourself 10 ms, sell them for 300k and watch everyone lower their price too.
The economy is perfectly stable it just favors the people who are more farmed and understand the supply and demand of the server.
Still waiting on that auction house though, pretty please?