But if you do decide to do something like taking all that time to make a rune arrow and then alch it, why not just sell the rune arrow and buy something more profitable to alch?
Because rune arrows sell for less than the high alchemy value.
It's the same reason that you don't buy a phat and alch it for money
Not even remotely close, only a moron would buy something worth millions to high alch it.
I only use high alchemy if the hi-alch value is higher than what I can sell it for on the grand exchange, which applies to most items.
Lemme give you an example then.
Rune arrows are currently 150 and alch for 306, meaning you get 156 more gold than selling in ge...but nope. the nature rune costs 179 gp.
You've made a loss of 23 gp by alching it.
Instead lets say you sell your rune arrows and buy something....say a polypore stick.
Its ge price is 40997 and it alchs for 45k. haven't you made a greater profit by doing that?
Now about the phat thing.
I may have misunderstood what you said then, but dont assume that the fletched bows and arrows are free. They may not seem to consume anything to make, but it will at least consume time. Time, which can be used for other activities that are more profitable or give faster xp.
If you take time efficiency into account or in other words opportunity cost, you will consider many things that cost gp better than things that are "free"
for example, crafting xp. Many people prefer playing the ithell harps over traditional crafting training methods, because they are "free".
But now think of the opportunity cost.
You might for example be able to kill roraris, and get about 1.5m an hour.
Making green dhide bodies is about 297k craft xp an hour, equivelent to about 6 hours of harps, but at the loss of 2214k per hour(rswiki) this means I am able to kill roraris for 2 hours, craft green dhide bodies with ge brought green leather, and still have 786k more than I started with, some more combat xp, a bit less construction xp and harmonic dust, and have an extra 3 hours that I can do somethign else with instead of the time used to play harps for 300k xp in 6 hours.