Customized means that they built the limbs to match your body's meat design, so your body can work with the limb's ability scores a lot easier. Standard limbs are "off-the-rack", so they don't work with your body, creating stress if you try to do too much with them without having support added as well.
I don't like the idea of customized meaning custom fit to a person. That's more of a Deltaware thing.
I'd prefer Custom Purposed.
So, getting a regular arm is cheap and common, because that limb was designed to replace a normal person's limb for normal use.
A customized limb is a niche market item created for specific uses. A surgeon's hand with extremely nuanced articulations for example.
Or even just security stuff. Nothing says bodyguard like huge beefy cyberarms.