Much has been written about custom harmonicas. I suspect that most players ignore it, and continue with off the shelf instruments. I was like this, until I tried a Joe Filisko harmonica. My friend, the New York harmonica player Trip Henderson gave me one to try. I remember playing one note, then bursting out laughing. I could not believe that a harmonica could be so powerful and reponsive.
The logic behind custom harmonicas is similar to acoustic guitars. You start on a cheaper guitar, then move up to a quality model, like a Taylor or a Martin. Also, many guitar players seek vintage instruments, because they sound better (I play a 1955 Gibson L50 F hole acoustic). The fact is simply this: you would never see a quality guitarist with a cheap instrument.
The same applies to harmonicas. Standard Hohners and Lee Oskars are worthy instruments, but they are mass produced. Hence only a short time is spent preparing each one. For quality custom harmonicas however, a master craftsman may spend up to a day preparing each instrument. This is why they cost more, and why they are so much better to play.
A key issue however is that, unlike a guitar, a harmonica will wear out. The answer to this is servicing. Custom makers will generally service their instruments, at a moderate fraction of the full price. This servicing should include replacement of worn reeds (e.g. the number 4 hole draw), retuning and re-gapping. The result should be an instrument which is as good as new (or better, if the maker has developed new skills in the interim). I own a number of custom instruments, some are going strong after many years, due to regular servicing. Generally an instrument will last me up to a year with regular playing. The net cost is about what I would have paid for changing guitar strings each fortnight.
Some players do go through instruments more quickly, and for them, the cost of regular servicing may be too great. However, as custom instruments are SO much louder than standard ones, less pressure is needed for the same results. Hence, a different playing style may extend harmonica life. For example, Jim Conway, the famous Australian player produces enormous volume and tone from his Neil Graham custom instruments. My understanding is that Jim has his instruments serviced about as often as I do.
Another aspect of cost is how many instruments are really needed. For Irish players, two instruments will suffice, as almost all tunes are either in G or D (or relative minor keys). For bluegrass fiddle tunes, I have four instruments, i.e. C, D, F and G. A blues player can generally get by with six harmonicas, i.e. A,Bb,C,D,F and G. Hence the cost of a workable custom set is similar to a middle range acoustic guitar, with the servicing costs similar to regular string changes.
In Australia, we are fortunate to have Neil Graham. His method is based on the Joe Filisko technique, which involves a specific contour being applied to each reed. Neil has travelled overseas on a Churchill Fellowship, to meet and study with the great makers and players. As a result, Neil produces instruments with enormous power and responsiveness. For example, they allow me to cut through loud Irish or Bluegrass sessions, while still having some in reserve. It takes a while to get used to this much power and tone.