In short: Good cashmere is worth it if you wear it often and look after it, because over many years it can cost only a few euros per wear. Cheap cashmere usually is not, since short, thick or recycled fibres pill and lose their shape after one winter. Check the feel, the knit and what the seller can tell you about the fibre.
When it is not worth it
Very cheap cashmere often uses short, thick or recycled fibres. It feels soft in the shop, but after one winter it can be covered in little balls and lose its shape. A good merino sweater often beats bad cashmere. And if you want something for the washing machine or the rain, cashmere is the wrong material.
When it is worth it
Good cashmere is soft, light and warm, and it lasts for years with care. Think per wear: a 400 € sweater worn 80 times over eight years costs 5 € per wear. A cheaper one thrown away after two winters can cost more in the end.
A quick check
Feel: soft, not scratchy.
Knit: dense and even.
Seller: can tell you where it is from and how it is made, with specifics, not just adjectives.
Next: what 100% cashmere really means and how to wash it.