My favorite yoga pants (which I'm wearing right now) are identical to Hard Tail roll over yoga pants, but they are "Bally's" and I got them at Sam's Club in a 2 pack (black on black & lt blue on navy) for $20. Although, this was 4 years ago...
Personally I'm not into the matchy matchy look of a coordinating top and bottom, but there is nothing wrong if that's what you like. I would probably suggest black or a color over white, since white will show any dirt and then it looks sloppy. Again wear what you like, but there are plenty of items out there that don't cost $100 each.