That obviously depends how you do it. There is a very basic attack spell that you can do for free, but it will be fairly slow. In the Fist of Guthix minigame you get some sort of generic runes, which you can use to attack your opponent for free, but that is also slow. As you use more advanced spells, using mind runes, chaos runes, death runes and the members-only blood runes, it gets progressively more expensive - but also faster - to use attack spells. Other options include superheating, and high alchemy. I believe in some cases you can do superheating (making bars) with a profit, too.
There is no magic planks in runescape. Only plank, oak plank, teak plank and mahogany plank.
depends on inventory and training
The magic tree seed costs around 300,000 coins. Search the item on the Grand Exchange to get the current price.
Look up a magic calculator, then how many times you have to plank-make from 86-99 magic, then add the prices of the planks + runes to cast the spell together, to get the cost.
Bump
It can do several things; read the announcements on the RuneScape forum. What I find especially interesting is that you get several uses of High Alchemy per day, for free, i.e., without runes (nature runes usually cost somewhere around 200-300 coins each). You do need to have the magic level, though.
no
approximately 3 million gp, if you are counting from 1 lvl.
There are farming calculators out there, and depending on your level and what you will be using to level this skill up you can get the price of that item and times it by how many you require for this answer.
The Battle robe top is a reward that can be purchased with tokens from the Fist of Guthix minigame. It degrades with use similar to the Barrows equipment, and may only be traded when completely degraded. It requires level 50 Defence and level 60 Magic to wear.The Robe Top costs 1,500 FoG Tokens.
A Skiller is a player who's Primary goal is getting all skills maxed out, basically getting 99 (120 - Dungeoneering) in all skills - whatever the cost.
an Ancient Staff on RuneScape costs 417k as of September.