Is monero still cheap to invest ?
Do dollar cost averaging.
Monero's long term trend is up, but its price is extremely volatile. That's why I suggest dollar cost averaging.
Let's say that you have x amount of cash that you want to invest in monero. If you spend all that cash right now on monero, the price could drop by half, and you wouldn't feel very good about that.
So pick a timeline over which you want to make your investment and do it gradually. You could buy a certain dollar amount of monero every week for the next six months, for example. Or, instead of caculating it based on dollars, buy a certain amount of monero as priced in bitcoin at a weekly (or whatever) interval on poloniex or bittrex, or your exchange of choice.