I find a simple analogy works best, "Something is worth what someone will pay for it." Those sellers are putting large price tags on cacti those particular plants because people are willing to pay that much. I guess partly due to the rarity but also due to the difficulty and time spent in growing a species such as Discocactus to maturity.
If you want cheap plants I suggest you find a heap of seed and grow them out yourself, you will get much more satisfaction in the long run and learn a heap in the process. The problem with purchasing a large mature plant such as the Discocactus you mentioned is you don't know how it was grown before you and occasionally the plant will not survive the transition to your growing conditions. Additionally, if you have never raised such a plant it may be difficult for you to care for correctly.
Here in Australia both plants you mentioned are quite rare and mature plants would fetch very much the same prices as you originally posted.
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