64GB is just fine as a starting point for basic model. I never went over 55GB on my 1st gen SE, that's with latest half year of my photos/videos locally, a lot of music and more apps that I actually need. After subscribing to Apple Music and a little bit of "optimisation" of apps, I'm currently on 40GB. With cloud photos I might even manage with 32 GB model, but that would be really pushing the limits and not worth it.
Anyway, 50$ extra for 128 is IMO fair price, if you really need it. For expensive pro models it is definitely correct that they start with at least 128.