I missed to answer to this...
I don't know. The current prices are part of a straight line. I basically decided what to be the lowest price and what to be the highest price, and then I got the rest from the line that connects both. The longer the period, the cheaper the price (for each 1 month). Now look what would be the prices for 1 week and 1 day: http://prntscr.com/cpi4sk I think they are relatively expensive. I also don't understand why so many people prefer the currently most-expensive price anyway ($19 for 1 month) 🙂
Also, to be honest, almost from the beginning I was thinking about rising the prices, but they are still low (in my opinion) 🙂