Years ago I removed the dust cover from my oracle Delphi and replaced it with a large plastic bag. The plastic bag did a better job of keeping dust out, but the primary reason I ditched the dust cover is it adversely affected the sound.
They tend to resonate and carry those resonances into the turntable structure and maybe that's why. I'd put a drafting weight (basically a leather bag with a couple pounds of sand in it) on top of mine while playing and that seemed to help.
I agree, $300 seems absurd but we are talking about a limited-production piece that is basically a big box of air while it is sitting in the warehouse, so a higher overhead item than one might initially think.
Also, all the ones I had, the plastic seemed to attract dust so I'm not sure they helped all that much.