I disagree. Depression is not a monolithic thing, and while there can sometimes be clear-cut medical causes, other times there are rather clear-cut external causes. Perceptions of the outside world, in other words, affect brain chemistry. Money won't always give you the perceptions you need to be happy, but it's foolish and (I must say) slightly insulting to imply that it can't be a major factor, or even a primary factor.
I'm dealing with fairly severe dysphoric atypical depression at the moment (which is more common than typical depression. Which is typical of the hackneyed insanity that is modern psychiatry, but I digress) and while I'm sure there are a number of factors in my personality and my brain chemistry, I fucking promise you that it is nothing a large infusion of cash could not solve. My depression stems largely from my persistent inability to solve the problems around me (which of course becomes self-reenforcing as the bullshit piles up and the depression saps my energy), but 95% of those problems would be trivially solved if I could throw buckets of money at them. It would still take time and effort, but believe me I would be tremendously happy while heaving those buckets of money around and waiting. Indeed, during those periods in my life where it looked like my career was going places and I mistakenly believed I would soon have, if not bucketloads, then at least a reasonable amount of money, my depression was at an all time low.
Depression isn't monolithic in its causes or its effects. The DSM V doesn't even begin to scratch the surface, and it should be noted that psychiatry in general is tailored towards people who visit psychiatrists, i.e. people who at least have enough money to afford transportation and health insurance. That excludes me, and it excludes the majority of humanity who are living in the third world right now. So, instead of relying on the DSM or whatever pop psychology definition is trendy these days, let's consider how they define depression in animal models: giving up, not seeking out food, not trying to avoid an unpleasant stimulus... apparently because they have lost the will or belief that they can improve their situation. Translated to human terms in modern society, this positively screams "money." And anyone who disagrees has almost certainly never been poor.