zakalwe2000 mentioned a lot of kinds of costs; you only mention the monetary one. In fact, your example of the iphone supports his/her point - a non-removable battery improves reliability and makes phones slimmer & lighter while also more mechanically durable.
Not everyone values slimness to an extreme, and despite the fact that non-removable batteries improve certain design margins, many many Android phones still have removable batteries, and some even reverted from non-removable designs to removable ones.
There is a crossover point wherein overhead reduction is "good enough" and when other factors about the phones begin to dominate.
The iPhone doesn't sell because of a non-removable battery, and the huge number of users who use bulky cases to protect fragile phones, and add-on battery cases, which increase weight and spoil the industrial design, tend to support the case that some people are willing to trade off thinness, weight, or cost for other capabilities.
I don't even think this is controversial. If you look at the number of users asking for microSD whenever a device removes it, or the people wanting additional clip-on lenses, there's a hunger to extend the phone platform.
Now, whether that happens over an integrated UniPro bus, an external lighting cable + case, or bluetooth, is an implementation detail of convenience, ergonomics, and bulk. I personally would rather have upgradeable camera modules or additional battery slots than "Morphie" cases and weird protruding clip ons that weren't designed to work with the builtin optics.
