Some of your argument is interesting, but the idea that something's value is equal to the effort that it takes to obtain/create the thing is certainly not the case. There are lots of things that are very difficult to create and/or duplicate that have no value. If I have my computer hash random strings until I get a hash that includes my name in it, even though it might take 10 hours to do (and would take another 10 hours to duplicate), it doesn't make that random string valuable.
Value is the benefit I get from having a good or service (http://en.wikipedia.org/wiki/Value_(economics)). While often times it is correlated with the difficulty in obtaining something, they are not equivalent.
That being said, your argument could still (sort of) work like this: there SHOULD be a cap on the value of a bitcoin.... the $ cost in computing power to mine a new coin. Whenever the price rises much above that, there should be an economic incentive to spend the money mining a new coin instead of buying the coin on the market. Of course, this price isn't a HARD cap, since there is still a capital expense in buying the hardware to mine the coin (or the opportunity cost of not using that hardware to do something MORE valuable), but it shouldn't get too high above that cost.
Of course, the fact that the cost to mine a bitcoin increases with each previously mined coin makes this even more complicated..