Roi should only be considered in terms of btc. If you bought a Jupititer when bitcoins were $70, you will most likely never make your hundred bitcojns back. What does it matter if bitcoins go to $1000? You would have had $70,000 if you hadn't bought the Jupiter, butif it only mines 50 btc then you have lost.
No. Only the price of Bitcoins at the time you buy the miner matters. Future changes in the price of Bitcoins more or less cancels out. The cost, in Bitcoins, of a particular piece of mining hardware is dependent on the price of Bitcoins. But after that, the Bitcoins mined can be compared to the cost of the miner in Bitcoins without regard for the future price of Bitcoins.
If I'm considering buying a piece of mining hardware that will cost me 50 Bitcoins today and can be expected to mine 70 Bitcoins over the next year, then it's probably a pretty good deal. The higher the price of Bitcoins, in fact, the better off I am with the miner because the lower, proportionately to the reward, the cost of electricity.