I wouldn't know about cabs, but you can get there via OCTA--the Orange County Transportation Authority, aka the bus and that's only about a dollar each way (including transfers unless they've changed things). I used to ride those buses all the time when I went to college down there. Yeah, they take awhile, but they get you there.
South Coast Plaza is nice, but very expensive (and w/ the US/Canadian exchange rate in the US's favor, it's super expensive for Canadians). I don't have to worry about that and I still won't shop there.