I am trying to get 99 magic, and I figured High Level Alchemy is the best spell to do it. Though, I need items that when the spell is used on it, I get more gold than the cost of the item + the cost for the nature runes. Do not include the price of fire runes because I have a fire staff.
First of all, let's see how many items you need to alch. Starting at 55 (the minimum magic level required for high alch) going for 99, gaining 65 xp per alch you need close to 198000 items. Alching takes 3 seconds, meaning 20 casts per minute or 78k xp/h. Thus, it will take about 165 hours to get to 99.
Also note that you can only buy 100 per 4 hours of most of the items, or even less of the higher valued ones, due to trade limitation set up by Jagex. These limits apply to members and free2play regardless. Consider this in your plans.
What items yield profit depends on the highly volatile market and especially on the nature rune costs. I suggest you use this automatically updated table as an indicator. Note that prices may still be several percent higher or lower, so some experimentation is useful.
The items I typically use include several kinds of battlestaffs, any rune items as well as what is cheaply available on that day.