Why are pentax lenses so expensive?

Someone gifted me a pentax k-r camera with kit lens. Its nice and I'm happy. I found out about bokeh and I think it's pretty cool. Prime lenses do it best. Right? And I don't have any prime kens which will let me do really nice portrait photography in which background is blurred
My friend got this lens for his nikon that was for 93£ which is 120$ I think. But now I'm looking to buy a pentax prime lens but they are so expensive, like nothing below 400$. Why is that so? And please suggest me a cheap lens If you know of any. Thanks.
Added (1). The lense which my friend got was a nikkor 50mm 1.8

Both canon and nikon make a cheap 50mm F/1.8. The quality of this lens is lower overall than their other lenses. Pentax doesn't make a cheap version of the lens. I have the 50mm f/1.4 from canon which is also around $400. You can look at 3rd party brands such as tamaron or sigma and see what they make for pentax. You would want at least a 50mm with at least a 2.0 aperture preferably a 1.2 or 1.4.

In photography, you get exactly what you paid for. Now that says a lot about Pentax glass.
The Pentax-A 50mm f/1.4 is a manual focus lens that goes for around $200 at eBay. You can use it with aperture-priority mode with your K-r. Just set to f/1.4 and the camera takes care of the shutter speed. Beware. After using this lens, you may want to throw away the digital 18-55mm kit lens. Don't. It's also a keeper, good for landscape and other general shooting scenarios.

The Pentax KR is a fine camera. Pentax lenses are fine lenses.
You don't get Ferrari cars for Ford prices and you don't get top quality lenses for budget lens prices unless you get a lucky find or are happy to buy Russian optics.
Look at an 80mm or 85mm F1.8 for a good portrait lens with which you can control the background focus far better than with a 50mm lens.
Mine are from Canon and Zeiss and a Russian 80mm F1, 8 which is as sharp as a new pin and has beautiful bokeh.
The sharpest lens in my collection of more than forty is a Russian Industar 50mm F3, 5 which is my standard copying lens.
It beats all my Canon Nikon Minolta Zeiss and Pentax lenses for sharpness and contrast and cost peanuts.

Are you aware that any Pentax "K" mount lens made since 1975 can be used on your Pentax K-r? The older ones will be manual focus but you can easily master that with practice. There are 4 Pentax manual focus 50mm f1.4 prime lenses now on eBay for under $200.00.
The Nikon lens your friend bought is the AF model which will not auto focus on a Nikon D40, D60, D3000, D3100, D5000 or D5100. The new AF-S model is about a $100.00 more.
A 50mm f1.4 prime lens will always be considerably more expensive than a 50mm f1.8 prime lens.