First off, I don't have a custom set. I do have a few over the ears (Shure 840, Grado HF2, Beyer DT770) and a pair of UE 5EB's. However, I did consider a pair of custom molds for a time.
For $170, I'd guess that'd be for the UE5. I wouldn't go custom w/o at least the UE10. Mold fitting can cost $25 to $50, depends on where you live.
First you need to decide on your goal. Are you going for total isolation or sound quality? If it's only isolation, I suppose any custom will do. If you want sound quality, choose the headphones first.
There are companies like Fisher Hearing or Unique Melody that will take your mold and make your existing IEMs into customs. Starts from about $80. Or go full custom like UE. Other vendors are:
JH (Jerry Harvey who started UE IIRC before it was sold to Logitech)
Go check the IEM section at headfi. Lots of threads on customs.
I decided to give up on customs, unless I sucked it up w/ the JH13's which are over $1K. I want the sound more than anything. I may end up w/ some IE8 or Shure530 or UE10, if I can justify the cost in my head. I'm losing that battle though...