I actualy have about 13k waters and only 600 or so earths, the problem is quite a few monsters drop a decent amount of waters (Green Dragons and Elves, I believe Basilisk may do too), Abyssals drop 50 airs at a time while Fire Giants/Greaters give you all the fires you need I can't think of ANYTHING that drops a good amount of earths and I'm always short on earths... The problem with combonation runes is that they're useless. You have a mud staff and a lava staff, that eleminates two of the runes. None (Well, almost none) non-combat related spells use two types of runes anyway so it'd be easier just to have a staff of one type. The only use the runes have is for pking, however you need to bring more with you at a time and they're more difficult to get, It'd be much simpler and cheaper to get say 500 earths and 500 waters than 1000 Mud runes.