I built my first in 1985, it was a clone of an IBM XT (Model 5160) with better performance. I bought all the parts at a Computer Fair. I believe it cost me around $1500 to build (I worked at IBM at the time and a gen-u-whine IBM XT would have cost me around $3000, even with my employee discount). The most expensive component was the 20MB hdd, a Seagate ST-225, which cost $500!
The IBM XT had an Intel 8088 cpu running at 4.77Mhz, my clone used an NEC V20 running at 8Mhz. The V20 used the same instruction set as the 8088 but was about 10% more efficient than the Intel part. That efficiency, coupled with the 8Mhz clock, made my XT clone about twice as fast as the IBM machine. The IBM came with a 10MB hdd, while mine had a 20MB hdd. My machine also had a single, 360K floppy disk drive and a whopping 640K of RAM. I installed PCDOS 3.1 on the hdd.
The one area where the IBM was clearly superior was the BIOS. My clone used an ERSO BIOS which was a very early Taiwanese BIOS that was very buggy. I bought an Eprom burner and some blank Eproms. I managed to "borrow" the BIOS ROM chips from an IBM XT and copied them, then burned a set onto my blank Eproms. I wasn't sure it would work but installed the chips on my clone mobo and fired it up. It worked great and now my clone actually reported that it was an IBM XT. It had 100% compatibility with the IBM, even down to the built-in Cassette BASIC interpreter. Early IBM machines had a BASIC interpreter built-in to the machine so that even with no OS, the machine would boot to a prompt where you could run BASIC programs. The programs could be saved and read from a Cassette tape deck.
The standard compatibility test for a clone in those days was whether it could run MS Flight Simulator. My clone ran Flight Simulator flawlessly and also worked great with Lotus 1-2-3 (MS Office didn't exist yet) and every other software I tried.
Since then, I've built many other machines for myself as well as for friends and relatives. I've never charged anyone for my labor, just actual cost of the parts. To tell you the truth, I would almost be willing to pay someone to let me assemble their computer since I enjoy doing it!
