Apple looks at every opportunity to hose its loyal fans and squeeze every possible penny of them. They make it difficult to get the battery and replace it.
That's the cynic's take, or possibly the take of someone who see's a conspiracy everywhere (very popular these days), but if you read some press releases from Apple, and a few tech notes, just about everything that some folks like to chalk up to avarice on the part of Apple (and, actually, I think that Apple makes plenty of money, so that they don't need to resort to squeezing and alienating their loyal customers), can easily be explained without the need to assume some nefarious goal.
The battery is probably the most important component of the iPhone. When designing a new iPhone Apple usually starts by looking at what sort of battery capacity is available with the very latest technology, and then designing around that. Size, style, functionality and features are all dictated by the battery.
Apple has to meet two competing design goals: to pack in as much battery as possible and to make their product as thin as possible. (The latter is a theme, as you can see for yourself, in every single one of Apple's products. Note even the latest iMacs, a desktop product, are impressively thin. Consumer research has told Apple that customers crave thin products.) So, Apple has managed to meet both goals by packing batteries into the iPhone that are oddly shaped, and by deleting design features that would make batteries easy to replace. It's a trade off.
But that's not the only reason that iPhone batteries are not user replaceable. If you make things easily user replaceable, third parties feel free to make and offer for sale knockoff products...like batteries. Usually made more cheaply. That's a huge problem for Apple, because modern lithium batteries have a very high power density, and if those batteries aren't made to a very high standard, and serviced only by experts, they can catch fire, and even explode. Apple saw this happen with early iPhones when users used third party parts (usually third party chargers), and it has happened much more recently (about a year ago) with Samsung Galaxy's. Having your phones catch fire and/or explode is a public relations nightmare for a company, and Apple is trying their best to avoid that happening.
So, yes, you may pay more to have your battery replaced by Apple rather than doing it yourself. But your phone will be stylishly thin and it won't ever catch fire or blow up. Is that a worthwhile trade off? You might want to ask someone who purchased a Samsung Galaxy a year or so ago and had it catch fire.