There is no actual price because diamonds can be in all different sizes, shapes, weight, etc. there is no approximate wholesale price or value for a diamond. Of course, there are a variety of factors that align to generate a selling price or resale worth for a diamond, as stated before, so you have your answer, there is no proper price.
Most items on skyblock.net can be sold for whatever price you choose, but its up to if the other person accepts the price. Diamonds can go for as little as 20 grass to as high as 30. There are no set prices on items unless they are in villager shops.
people complain about it because it will kill the "economy". Not a good idea to upset the people who love making net an economy driven server.
Its not "economy driven" the want to keep the already unbalanced economy balanced. All trade is economy. The server has an economy. If you ever suggested that, prepare for an onslaught of angry skyblockers. Terrifying ¯\_(ツ)_/¯
