Yes, I've used the REL RT's. They have a slight hardness to them IMO. I'm sure the Sonical Platinums are better (I do use these elsewhere without a bypass), as are the V-Vap teflons. But better yet I absolutely love the Dueland Alexandra's as a bypass (haven't tried the CAST but that would be the cat's meow).
Sorry, I know you were looking for a cheaper alternative, but there you have it. Perhaps the Clarity caps might be worth a go.