Well it's always better to invest when the price of Bitcoin is low but we cannot predict what will be the value of it by tomorrow or in the following days. You can study the graph/trend if you want so that you can estimate if you think it's the right time to invest. However, I believe that Bitcoin's price is more on increasing than decreasing so I suggest start investing now because its value will still continue to rise as years pass by. Invest now to earn profits already.