Well, when I sell things on the ge I normally drop the price 5 - 10% to sell it faster. Certain things in the game's economy are pretty stable, like the generic money-making methods of skilling: iron, coal, yews, lobsters, etc. unless a large number of people(merchanting clans, for example) are buying/selling a certain thing on large scales. The gems are another fairly stable item that's used commonly by thousands of people training skills and producing items for profit, so normally only one type of gem at a time will drop in price and it normally doesn't stay too low. 50 thieving might take a week at the max, depending on where/how you train it. My advice: Get a paypal and link it with your bank account so all your funds get dumped into your bank and so you have access to the funds immediately on a debit/credit/ATM card type thing and so if some asshole tries to dispute a payment on paypal, paypal can't lock up your funds and keep your money from you. Worst case scenario, you have to withdraw a ton of cash to keep it from paypal. I'm not sure how much iron or coal you'd mine per hour, but I think depending on the location and server population, you could get easily 100k/hr in cash per bot. The level of whatever skill you're using to bot really comes into play though, so if you want to start this business idea you should take a month or two preparing to start it: Get the accounts, get the paypal/bank account set up or however you want to handle funds, save money for investing in this(You need members for each account, depending on what you're botting. You could start off with an f2p skill/activity and eventually upgrade each account one at a time, training the thieving while you bot the others in f2p, then set the one(s) with members to work in rogues' den.), you need to buy the VPS, and whatever other expenses you're going to need. Basically I think you should prepare, save up, then invest and launch your business. You could go a step further and have a webpage store for automating transactions or something, hell, I don't know. I've played the game since 2001, on and off, so I know what the in-game economy's been like. It's slowly inflated. I remember when the lowest price for 1m was around $20. Meh, just check out all the skill training guides and then check item prices and fluctuations in prices using the new grand exchange charts on the runescape website. Whatever calculates out to the fastest and simplest gp/hr is what you want to bot. Hell, if you really want to play the economy you could stock up the items you farm in large amounts and sell them all when said item reaches a peak in price. I should probably stop typing, eh?
If you want more advice, just pm me or something. I type too much. :/
Oh, another thing: Don't get sued, arrested, etc. If Jagex catches on, they'll likely have a small shit-fit in their legal department. Just don't advertise in-game for a website, that's the #1 way to get caught and legally raped.
It'll cost you more than $600 initially if you buy all the memberships, VPS, and other stuff right at the same time. It's around $60 for 10 account memberships if you pay via credit card.