Maybe Kimber 4TC? The 4 is about half the size of the 8TC and sounds pretty good. Many strands without large shielding make it fairly flexible in my experience.
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I'd second the Kimber suggestion above. If you need really long runs, then the cost could go above $200, in which case I'd recommend Kimber 4VS (essentially the same cable at 4TC, except it uses a polyvinyl insulation rather than Teflon). In a bookcase system, I seriously doubt that you will be able to detect any audible differences between 4TC and 4VS cable.
I've never heard them, so take this for what it is, but I've seen speaker cables made by Totem Acoustic (the Canadian speaker manufacturer) that look very flexible, and I believe they're in your price range.
Once again, I haven't heard these, but Totem makes high quality product and it would be worth a look.