But beyond mining cost what dictates the price is simply the demand for it. If there isn't demand, the price would drop, ultimately is all supply and demand. Again, you are also ruling out bribing existing stakeholders thus being replaced with these new stakeholders with evil intentions. These are fat fetched scenareos but not by definition impossible.
mining costs and those buying from miners have a valuation right now of $500-$550, beyond that... the price($550-$650) is the speculation part (S&D):
speculation is the profit and variance factor, which can move the price down or up due to demand or excess supply.
imagin it this way..
baked beans..
the whole sale price (cost of production is 10c) speculation is the retail price that is variable. so if there is a supply excess the retailer will do special offers, "buy one get one free" for instance. but the offer price will always be higher then wholesale.
so imagine the whole sale again is 10c a tin of baked beans. and retails normally at 50c, excess supply he offers 2 tins for 50c meaning 20c cost and still 30c profit, or he will just reduce the price from 50c to 40c, or if its near the 'sell by date' he will sell for 15c. in each scenario he wont sell at a loss. however in a time where there is huge demand he can raise his prices.. maybe up to a dollar.. either way the true value of baked beans is 10c and anything above that is speculation/profiteering.
now back to bitcoins
in december the mining costs were around $350-$400 but speculation (profiting, S&D) brought the price upto $1200 due to ALOT of speculation(china buying a crap tonne of coins and alot of positive news). then when bad news came about(gox/china government ban/FUD), speculation died and the price tumbled back to the true value of around the $400 mark.. check the charts and you wil see the mining costs track the average low's of each month.
the high prices are pure speculation. so again even if someone tries to dump below the mining costs (true value/low), then it will just by nature retrace back up to the low price, and then speculation will kick in to bring price back into profit. after all smart people wont sell at a loss and eventually the 'numpty' (dumb) people selling at a loss will run out as thy are a small minority.. meaning there wont be any sell orders at losses.
now to think about bribing stake holders.
lets say a government bribed stake holders by offering them $550 into the bank account, if they sold their bitcoin way below $550.. smart people would not sell at a loss so the government would have to offer $550+ just to persuade hoarders to sell their coins at any price less then profit.
so now imagine the price was $100.... those ex-hoarders will then use the multiple lumps of $550 they have in banks from government bribe, plus the dollars in the exchange from selling at a loss. to then buy like crazy, all the bitcoins that are priced below $550, simply because the end result is they would have more bitcoins. (end result price rises back to over $550).
so again any bribing or intentional price dump will retrace back up