Sounds like the free market at work, so I'm not sure "ripoff" is the right term. The editors have bet that they can pay this paltry--by your standards--amount to content providers, and still attract a quality of content sufficient to keep advertisers and subscribers happy and the magazine financially viable. The marketplace will pass judgment on this decision soon enough; if enough readers and advertisers agree with you, goodbye magazine.
I expect that they'll be able to get all the portfolios they want at just about any price they're willing to pay. Those submitting portfolios obviously perceive some value to the exercise beyond the strictly monetary. Again, the free market at work.
What's the trouble?