K-series motors still cost quite a bit. They also require more parts and effort.

You can swap a B-series into your car using entirely OEM parts--its as simple as can be. With the K, you'll need an aftermarket mount kit, custom axles, either a custom wiring harness or the know-how to wire it up yourself, a custom header and probably some other junk too.

The main advantage of doing a K-series at this point is that its cool... As K-series motors drop in price, and the aftermarket products necessary to use them become more available, it will be a more feasible option for the average enthusiast.