How much do generic alchemical or magical components weigh?
As a player, if you were planning on doing some alchemy/magic item creation on a journey, you'd have to buy the prerequisite materials before you left, which means if you're working on a +10 magic sword, you've got to be lugging 100,000 gp's worth of materials around with you. Alternately, as a GM if you're giving out 'magic item creation components' as loot raided from an enemy wizard's lab, they need to be able to cart that stuff out.
Long story short, I can't handwave it, so I'm wondering if there are rules/traditions/remnants of old editions that might help me answer this question without just assigning an arbitrary number.
It's going to be arbitrary and variable. Crafting components for most magic items are nebulously specified, defined solely by their value. 30,000 gp worth of components for a magic ring might have a completely different weight than 30,000 gp worth of components for a cauldron of flying.
I knew a GM who decided that gold was the solidified essence of magic. So the 30,000 gp of components to enchant that ring or cauldron were 30,000 gold coins which the crafter would slurp the magic out of to empower the item.