My friend and I (Darkbob50) were trying to find a way to make money yesterday, and we thought about mining clay- it is, after all, very quick at at least 50 Mining, and it's worth about 100 GP. But when we were looking up how much it was worth, we noticed that soft clay was worth around 70 GP more. Searching a bucket of water, we found out that those were only 4 GP. Putting the two together was easy.
There are two ways to use this strategy:
#1. Merchanting: This involves a lot more buying but is a lot faster than mining the clay. You can start off with about 100k or so and then buy 900 clay, spending the remaining 10k on buckets of water (you'll get about 2500). Set your "Withdraw X" amount to 14, then withdraw 14 clay and 14 buckets of water. Then, of course, use the clay on the buckets and on the pop-up screen, select "Make All" soft clay. Continue this for the rest of the 900 clay, leaving 1600 buckets of water behind. Sell the soft clay (you should get a noticeable return), and use that money to buy 1600 or so clay. Repeat the whole process of making wet clay. This time your materials should run out. Use the money from the soft clay and buy more materials, repeating the process until you have the amount of money you want.
#2. Mining: This way requires a lot more time but a lot less spending. Go to Varrock's west bank and empty your inventory except for maybe a few energy potions or other run-energy enhancing items. What I would wear to do this is anything that doesn't have weight, like random event clothes of Boots of Lightness, if you have them. Equip your pickaxe and run south, following the Varrock wall. When you hit the mining site, there's three or four different spots of clay. It's a real short run back up to the bank, so you should be able to get a lot of clay per hour. This method saves the money from #1 that is spent buying clay. All the money you have to spend is for the buckets of water, which are only 4 GP each. So if you mine five thousand clay in a matter of days, it's only 20k to buy the buckets, and then you'll get a return of around 820k!!!
I've resorted to this method from my previously-bedloved "steel bars" method (see the topic "The Most Awesomest Way to Make Cash") because I realized that my calculations on that matter were entirely off. Instead of 1 iron + 1 coal = appr. 280 GP, with 600 GP/steel bar, the equation is actually 1i + 2c = 460 GP, which means that you're only making 140 GP per bar- a fairly measly amount for making mass cash. I've also started to use it because, as you might have seen on my Twitter feed, I recently got killed on a PKing world (skulled) and lost over 1m of F2P stuff, including a rune platebody and platelegs (g), the crown of my entire account, so now I've been looking for a cheap and fast way to make good money not only to repurchase my rune (g) but to get the full set and get some money for when I (in 4 months) become a member again.