I suspect I'm replying to Kebabbert, which never goes well, but here goes...
First off, these systems are for companies that are already in the ecosystem. iSeries users tend to have minimal compute needs (a huge percentage of the iseries installed base is on one-socket Power5 machines; some are on older systems) and a single-core iseries-capable P8 lets them get a fully-licensed turnkey machine for a relatively low price, while still getting a performance upgrade over legacy machines. They can then keep up with newer OS releases and the like. In general, folks moving to a single-core iSeries machine are not going to be greenfield customers.
As to Power8 in general - we run Linux on Power8 because the perf/$ is actually pretty damn good. 8-core S812LC starts under US$5k. For our workload, this Power8 CPU config beats an Intel Xeon E5-2650v4, partially due to the much larger caches and extremely high (over 90GB/s STREAM Triad!) amount of memory bandwidth. There's also some really nice edge features (memory compression) that are kinda cool, even though their support in Linux can be kludgey - for instance, Power8 memory compression basically runs as a hardware assist to zswap, as opposed to on AIX, where the system actually sees more available RAM. We'll probably grab Power9 too, since early performance estimates look superb, depending on whether IBM improves their horrendously unpleasant procurement process for OpenPower systems.
I'm not saying the Big UNIX stuff is competitively priced with x86, because it isn't - but then, neither is anybody else's. But for OpenPower machines, pricing really isn't terrible.