The Ikea MARKUS swivel chair is the most comfortable "desk chair" for the price point. I've had a cloth one for 3 years now and the support on the mesh backing is still tight and the adjustable support bar is still nice and springy. There are definitely better options out there but this chair beats any Office Depot/Staples chairs out of the water, and it's pretty stylish as well in a Euro-minimalist kind of way.
I sat on the Mirra, Aeron and Embody, all from Herman Miller. I also have the IKEA Markus at home. This last one is very good for the price, but I don't know if IKEA gives discounts.
From the HM chairs, I was really impressed with the Embody. It fills and supports your back completely. It also has some freedom of movement, so you can turn around a bit and it "follows" you. I liked both the Mirra and the Aeron. For me there was little difference as to comfort.