You should probably stick to high alching Yew Longbows, since there's always people selling them on the Grand Exchange, and they yield the most efficient xp per item and gp loss per item.
The anwer above is generally considered the best choice if you buy all of your supplies and want to minimize money lost.
If you want to take your time, and make money, this is my approach. I make all my own runes, and buy pure essence with profits made from making death runes. This makes rune cost essentially just the time it takes to make them. Not a big deal since I need the XP anyway.
Having done "Hand in the Sand" Quest, I get the free sand almost every day and also buy 80 seaweed (2 gp each) from Arhein at the Catherby docks almost daily. I use Lunar Magics to make the molten glass after I've accumulated a couple months worth of supplies.
After I've blown and charged air orbs (using cosmic runes that I've made), I make air battlestaffs using plain battlestaffs purchased from Zaff in Varrock. After the Varrock Achievement Diary came out in September 2007, the level 3 Varrock armor allows you to purchase 64 battlestaffs each day. The battlestaffs cost 7K gp each and the air battlestaffs high alch for 9.3K gp. Since the diary came out, I've made and alched at least 30k air battlestaffs and made a good 75 mil gp or so. This plus level 95 crafting and 97 magic.
It takes 20 minutes or so every day to gather the supplies, but is very effective over the long term.