14,500,000 gp
http://services.runescape.com/m=itemdb_rs/Dragon_platebody/viewitem.ws?obj=14479
If you mean Platebody, it's around 10 Million in the Grand Exchange.
The price of the dragon plate currently is around 10.6M, but like anything else expensive, prices change.
500k-600k Not too Much Its not good without The full Set. Save
The full rune armour set was 300-400k but now its less then 200k. A rune platebody cost about 60-80k but you can buy it from Oziazh for around 65k but only after dragon slayer.
they cost 200k.
a lot
£3.50 per month from credit/debit cards or £5 by mobile phone. there are many methods, follow this link - http://www.runescape.com/members/members.ws
They cost 19.95 because you get all six and you keep them.
A whole lot.
10m
a lot
You get exactly 187.5 xp per steel platebody in runescape which is probably the best way to raise smithing cost wise