Are they shared servers or just low quality (what exactly is a low quality server)?
How are they cheaper than mainframe servers?
They are servers you can buy in normal shops. not loq quality, but not special build, not somthing that is hard to get.
The complete range dfrom Dell, IBM is commodity, as are Tyan, Supermicro. I would not call them low quality.
Basically, in my definition, everything for which you can buy the individual parts IF YOU WANT (like Supermicro - all elements are available independently, and can fully interop with other manufacturers, not like mainframes).
Well, given that "normal AMD / INTEL" servers stop scaling about pretty much a LOT lower than a mainframe start - sure they are cheaper. Also a lot less powerfull. Did IBM not run about 10.000 linux VM's in a Mainframe about 8 or so years ago? Try that today with a highe end Intel - will not work.
Nowadays most servers you would by from Dell or HP or Compaq or whatever supplier you choose, would be classified as a commodity server. The word commodity here simply expresses that they are built from freely available parts and based on open standards. This is opposed to mainframe servers, which are generally not based on open standards and often use components only available from that specific vendor (e.g. IBM A/S360 systems or similar).
They are cheaper because the competition on the market forces prices for the components down. This does in no way mean they are not good servers. On the opposite: the same market competition also forces suppliers to keep up with good quality and improved performance, otherwise they go out of business.
Plus: as an end user you generally get lots more hardware for your money, which you can then use to build massively parallel systems, which generally will beat the crap out of any mainframe (unless you have very specific computing requirements). And if any one of your cheap commodity servers goes down, your system as such (if well designed) does not.