The price of the masternode coins is very volatile in a short amount of time, so the actual price of IQ coin and its passive income from masternode could decrease easily during the coming weeks and months.
Price of any asset is changing everyday because it is being traded. However no masternode coin is going to become the next bitcoin as far as price is concerned. So you might see most of them actually depreciating their value like anything and after some months you will find yourself holding a couple of masternodes which have no value any more while bitcoin is going up like anything.
The best method to make a passive income is by holding the most valuable cryptocurrencies, like Bitcoin, Ethereum, for several months or years.
You will get that passive income if you sell and buyback not just holding them. Compared to masternodes they are better obviously.