Plasma is a bit of an outlier in terms of price. Vendors have no choice but to compete on price because while plasma's typically have great PQ and black levels they also often have fucking ugly bezels, are heavier and still to this day use more power than comparable LED/LCDs. They have a negative stigma due to image retention and burn-in as well, many people think that they can't game on a plasma because the UI will burn in for example. Educating the public on that stuff takes years so its often easier to just break down the prices. That's mostly at the mid and low end of the market though. If you look beyond the entry level 3D plasma stuff, it's still very expensive.
Playing: Dark Souls 2: Scholars of the First Sin Watching: Mr. Robot