Fletching is pretty much free and I did it in 2 weeks.
I got 99 magic and 86 smithing at the same time for less than 3m gp.
Cooking is supposed to be cheap as is firemaking.
The list could go on because most skills are easy/free one way or another. Just depends what you fancy going for ;P I went for fletch/magic because I love the magic cape and wanted it trimmed.
1-70 Costed me a small amount just following a guide.
70-99 I bought yew longbow (u) and bowstrings, stringed them and sold them. Did it this way because at the time (maybe it still does) it made me a small profit and stringing is quicker than fletching or fletching+stringing.
You can make money with 99 Cooking. 99 Firemaking cost around 4-5m with Maple Logs. 99 Fletching costs maybe 10m (I think) with Maple Longbows. Thieving is also a quick one, if you have certain quests done you can make a good profit and a very fast cape.