Question:
How many minutes of calls and number of SMS do you make/send per month?
How many minutes of incoming calls do you get per month?
As for whether 100 MB is enough, yes it depends on your data usage but quite a lot of people here forget to mention that it also depends on your phone's data usage. Smartphones use background data and the more apps you have installed, the more background data it can use. Even without installing any new apps, phones already use background data.
Yes, you can restrict through the settings, but many may not realize until their tiny 100 MB cap has been sucked up in two days by some background update. Or that video they opened cost them 50 MB data in a few seconds.
For me, at least 1 GB is a safe bet and provides a lot of cushion, even if you're a light user. And if your call volume/SMS usage is it's better to go for prepaid because it's very cheap and you can roll over your unused data. At 100 MB per month, the 1GB you bought at $7 can stretch from Jan to Oct at a total cost of $50 (or $5 per month).
Personally I mostly use WhatsApp, other apps like iris Next Bus, Maps, banking, etc, and some of website surfing. I don't watch videos on data (only do on Wifi), and any large downloads I do over wifi. My monthly usage is around 500 to 700 MB.