In general, the harder it is to obtain the item in bulk, the less you lose when alching it. For example - Rune Daggers alch for 4.8k each, and their trading price is 4500 on Zybez. However, you won't really ever find anyone selling more than just a few Rune Daggers at a time. On the other hand, it's very easy to purchase or produce thousands upon thousands of bows, but the losses on those are almost as much as the nature rune cost.