I didn't watch it enough to ever answer the question how a columnist could afford her apartment and wardrobe.
Well, her apartment wasn't exactly huge, and the conceit is that she'd been there since the 1980s and it was a rent controlled apartment - wikipedia claims there's some dialog saying she's been in the apartment for 20 years at whatever point in the series she says it. I've never lived in NYC, so I have no idea how true to life that all is.
In fact, her apartment and wardrobe figure in an entire plotline when one of her relationships ends and she may need to get a new place or buy he apartment back from her ex (who had bought it and the one next door from her landlord I guess) and she realizes she's actually broke (as in no savings whatsoever) and does the multiplication in her head and realizes she's spent more than the down payment she needs on her shoes. Miranda, the practical one, of course, is the one to point this out to her.
Later in the series and the films she's turned her column into a series of best selling books, writes for Vogue (maybe in addition to her newspaper column, I was never clear on that), and of course, marries fairly well. So money, at that point, is definitely not a problem (though she never gets rid of the apartment).
I loved the show and think it has one of the best series finales ever. I still think the first movie is rather superfluous given the way the series ended and just retreads old themes explored enough in the series. It's still fairly well written though, so I can enjoy it.
The second film is absolutely horrendous though, and I can see why it took them forever (and money) to get Kim Cattrall (Samantha) to agree to do it.