Why is earth runes and water (37 gp each) more expensive than fire or mind runes? I'm just thinking. Some reasons; Water runes are used for OSRS gold blitz/barrage which is extremely popular in P2P. These runes are not very common and can only be crafted by very small numbers of people.
Fire runes are frequently dropped by many creatures. Mind runes can be quite ineffective. Also bear in the fact that air runes are the runes that appear in standard spellbook spells the most frequently. The reason body runes cost so little is that they are nearly ineffective. Earth runes are employed for the Vengeance spell, which is 10 at a time, and this spell is all over p2p and pking.
Although I haven’t played for over one year, I do remember that the fastest method to earn money from combat in f2p was by collecting large bones. This gives you around 30k/hour. There aren't any NPCs at high level that give rewards drops. It is possible to get around 10-15k from the lesser demons. The F2P player would then take about 3 hours to collect 100k combat drops.
P2P players have a lot of choices. They could earn up to 100k per hour by gathering dragon bones and green dragons. They could form teams to hunt the KQ, KBD bosses and GWD bosses, and attempt to get a 1m+ drop. There are many ways to cheap RuneScape gold make money in P2P.