Assuming you will be selling the gold you make with the bot, here is how to figure out if it's worth it:
# of days using the bot = 14
# of hours botting per day = h
Gold/Hour using rc bot = g (depends on your level/method you use)
Price you sell rsgp per mil = p (usually $.35-$.45 per mil if selling to a trusted buyer)
Money made in 14 days = 14*h*g*p
Just plug in the values for h, g, and p, and if you make more than the cost of the bot ($13), then it was worth it
For example, botting 12 hours/day, making 200k gold/hour, selling at $0.4/mil would result in $13.44, which would pay for the bot.
EDIT: Also, take into account the fact that after you are done with that order, you will be able to bot your main to 99 rc and have your other rc account botting gold at the same time. In my opinion, it's definitely worth going ahead and getting the 2nd script now.