relative cost 300 vs 55-300 zoom

The 300mm is heavier, professional construction and is built to last many years of hard use. It is also a relatively fast f/4. The 55-300mm is of lighter construction, not so highly corrected and with a more limited maximum aperture. Having said that, it performs extremely well.

If you handled them side-by-side the differences would be very obvious. This isn't meant to denigrate any lens using large amounts of plastic though, because they can be capable of excellent results.

There are different types of aperture mechanism in zooms as well, is that right John. My understanding is that a variable aperture zoom (eg F4.5-5.6) is cheaper to make than a constant aperture mechanism, although the bigger glass diameter usually needed for a constant aperture would also add significantly. The cheaper variable aperture zooms also have less precise Fstops at different zoom settings I believe.

We take our pick really .... The slightly lessor IQ and more budget build of the zoom are offset by the lack of flexibility of the prime. I'd certainly find a 300mm prime difficult to use without being able to make quick framing adjustments ... At that sort of length then it can be more of a problem with a subject too close than too far, especially moving things. But a 70-300mm F4 zoom? That's going to be quite a beast and expensive, so we're left with the more budget choices with smaller max apertures.

The new 150-450 for full frame is quite a game changer I think, although not priced for Pentaxs core 'enthusiast' users I fear.