choosing a FPGA from 28 nm technology

If all you are buying is single chips, then something like digikey prices will be what you pay. But the smallest cyclones start at around $30, but thats the part price. You probably want a dev board that will have discounts to academic institutions.

As for how to compare things to xilinx, that will be difficult. I assume you'll need the same Input and output, so IO requirements will be known. Power comparison should also be pretty straightforward. But as the Altera and Xilinx architectures are different, it is very difficult to compare area of logic or DSPs.