Why are MOSFETs so expensive?
I am not sure what you mean by 'typical' MOSFET, but prices of many types of components can vary. It also depends on what you class as 'expensive'

You can get some very high priced components as well as very low priced ones. It depends on a few things (from my experience) such as package, durability, accuracy, how many you purchase, operating temp, application and even 'special features'. Take for example this MOSFET; the IXFB100N50Q3. Pretty unremarkable really when you look at the stats compared to most, but this one costs £23 for 1!! Now that is pricey! If you look at the power dissipation though, you can see that it has a Pd of 1.56kW which is huge. For some applications it is definitely worth that money! But, even though it is £23 for one, if you bought over 1000, they are £16 each. 

Now for another MOSFET, on the other end of the scale, the 2N7002 which I have personally used alot, because it does the job I need it to, and it is less that 10p!! Buying over 1000 of these at a time makes them almost 1p each. That for me is a 'typical' MOSFET and I would not call that expensive!

Now for microcontrollers.... These can really vary in price, from very basic 8-bit microcontrollers like the PIC16F1824 which is around £1 to but, or if you get them in bulk, about 76p!

On the other end of the scale, an all-singing, all dancing, full-of-features such as the ADUCM310BBCZ costs a whopping £42.50 each! Or again, bring the price down to £35 by buying in bulk. 

So, you can't really make a claim that 'MOSFETS are expensive', you just need to shop around and look through datasheets to find one that is in your price range and suits you.

NOTE: All I did was searched MOSFETS and MCUs on Farnell and sort by price to get this data. I am sure some more experienced people can come up with a more in-depth reasoning that I can as to why prices vary so much, but I hope these practical examples will be a good starting point.