subscription fees + time invested in leveling = sunk costs<p><a href="http://en.wikipedia.org/wiki/Sunk_costs#Loss_aversion_and_the_sunk_cost_fallacy" rel="nofollow">http://en.wikipedia.org/wiki/Sunk_costs#Loss_aversion_and_th...</a><p>players will spend more time playing than they would otherwise want to due to their perception of sunk costs, hours of gameplay provided expands to meet this demand with a grinding mechanism being the cheapest type of gameplay to offer