One can often find top-quality sights used for very good prices, check Ebay, gun shows, your local trade paper, etc. I have picked up several for less than $100. Either Diana or Anschutz match sights will fit most Diana airguns.
You can easily solve the "problem" of low-light use of aperture sights simply by using differently-sized aperture disks, or better yet with an adjustable aperture. Peep sights are in fact MUCH better for low-light situations if the opening size is appropriate to the conditions.
With sporter match sights like the popular small Williams units, a popular trick for late-day shooting is simply to remove the aperture disk, and sight through its mounting hole! You can still shoot with remarkable precision that way.
Here's one of my favorite set-ups, a Weihrauch rifle with an old Anschutz sight and a Gehmann magnifying adjustable iris. I have less that $150 invested in these two accessories, and of course the iris can be instantly "transplanted" to any similar sight.
