Sure, I can find out what the cheapest possible mac is easily, but how is it as a development machine? Is it powerful enough to deal with all the involved apps? How is the overall user experience from the dev side? Do you wish you had paid more and gotten a MacBook?
For that matter, maybe I should go on ebay and buy a used, older generation Mac instead?
Being satisfied with your development environment is about more than quantitative considerations. Asking this sort of question in a place where a lot of Mac-using developers congregate is lazy? I don't think so.
So far objectiveC seems pretty crazy, coming from Python at least. It seems like the opposite of pseudo code if such a thing could exist.
however, if you are looking to learn to develop for iphone (as you mention above), then I too would go mac mini before apple ditches them.
when you add this in, you maybe could swing a refurb imac or macbook from the apple store