You have to admire how subtle and – in some way – kind is Apple on making you spend money, and then some more money, on their products.
I took a 64GB white iPhone 6 this last December, and managed to have all my data successfully back-upped over iCloud. Yesterday I had a warning of low space on it, and after a check I found I had only left 180 MB.
I don’t use any of the Streaming Photos feature, and at least for now I am not a heavy user of apps requiring lots of iCloud Drive space. The apps eating up my space are actual photographs on my device (something near 6 GB of data) and 1/2 GB eaten by Whatsapp [in my family we use it a lot, and we enjoy making photo and vocal jokes with my very young nieces].
I was a bit confused by the how iCloud showed me how actually I use its space. First it tells me it needs ~4.4 GB of data to backup my iPhone, then another ~ 800 MB to Whatsapp and other iCloud Drive files. Digging into the iPhone backup I saw again Whatsapp, this time with more space used, and then the apps on my phone.
Thinking about an incremental backup (a la Time Machine) I deleted my backup and started it anew. At the end of the process I gained only 350 MB of space.
So, seeing how somewhat limiting myself with photos I already took over 6 GB of my phone which had to be saved in my backup I made my mind and moved to OS X iCloud Preference panel to buy the cheapest upgrade to iCloud Drive.
Per se it’s a small investment, so I can afford to set up and forget it. But at least I have to admit defeat to those evil marketing strategies by Cupertino …