But we could also buy it for 6M RSGP through the GE right? And that would give me 14 days membership?
A bond, is something a runescape player buys with real life money.
This bond can be sold on the GE for runescape eocgp.
People buy bonds mainly for membership, but you can also exchange them for spins, name changes or runecoins.
To make a bond tradable/sellable on the ge, you need to pay 10% of it's ge price. Meaning, if the you bought it off the ge for 7m eoc, it will be untradable (you can exchange this for membership for yourself etc. To make it tradable you need to pay 10% of the ge price, meaning 700k in this case and it can be re-sold to the ge or sold to other players via trade screen.
1 bond will get you 14 days membership, and will increase at the same rate;
eg 2 bonds will give 28 days membership.
If you have runescape gp, you're better off buying membership with bonds as 2 bonds only cost 14m rs3gp (taking the assumption that they're 7m each), and 14m rs3gp is worth 4.2$, whilst traditional membership paying with paypal etc is 9.99$/month.