How about:
- Find a quad core system under $800
- Find a quad core Nehalem system with 12 GiB RAM under $1200
...and there are lots more.
The chick in that ad wouldn't know what a quad-core processor is if one bit her in the ass. Again, she portrayed the typical demographic to which the PC manufacturers cater with their lower-end offerings: the technologically-ignorant/apathetic "Wal-Mart shopper."
As someone who works for Wal-Mart, here's a little story: A few weeks ago, a customer came in and asked me, "Where's that computer you had for $499?" We have several different computers for that price, laptop and desktop, so I asked her which one she meant. "I dunno, it was a laptop." Again, I ask her to narrow it down a little. I even go with her to the laptop display and open it up to see if I can refresh her memory. I asked, "Do you recall which brand it was?" and the customer said,
"I dunno...I don't care about the brand, I was just looking at the price!"
That one phrase pretty much sums it up. Wal-Mart shoppers will buy anything as long as it's cheap and shaped like a computer. Obviously, the customer didn't do any research whatsoever before buying. If she wouldn't have stonewalled about the price I would have tried to help her by asking questions such as, "What will you be using it for?", "Is anyone else in your family, say kids, going to want to play games on it?", "Do you have broadband or dial-up?", etc, in order to determine which system would best fit her needs. Instead, because the customer
just didn't care and wanted to save money
so badly, she went home with a underpowered 15" Dell with integrated Intel graphics, 1 GB of RAM, a 120GB hard drive, and a 1.83GHz AMD processor. With Vista. Yeah, that ought to be fun.
So in answer to AidenShaw, no, of course you can't get a quad-core Mac for under $2,500. Nobody's worried, because Apple doesn't cater to the above mentioned demographic, who by the way, don't care about quad-core thingamajigs anyway.

Mi¢ro$oft can pull the old "economy is so bad, look how cheeeeeep you can get our OS" all they want, but again, "Macs are too 'spensive'" has been thrown around since the mid 1980's. Who cares.