>>> how much would I expect to pay per month . . .
I'm still getting to grips with how much I need to pay but precisely because of that (i.e. because I've been monitoring my data usage etc) I might be quite well-placed to advise you, GG.
I bought a SIM-free phone (allowing me to choose my own provider) and put an Asda Mobile SIM into it (because they offer one of the cheapest services around).
I then used the calculator here
https://mobile.asda.com/bundles/calculator
to work out which 'bundle' might be best for me.
I opted to pay £12 per month (or, more accurately, 'per 30 day period'), which gives me 800 minutes of calls [which I'll never need], totally unlimited texts and 3GB of data. (See here for all of the available options:
https://mobile.asda.com/bundles ).
I also set things up so that the £12 comes out of my bank account automatically every month. Further, I configured the wifi connection on the phone to use my home wifi, rather than a phone signal, whenever I'm at home (so it doesn't cut into my Asda Mobile data allowance).
I've been deliberately using the internet quite a lot while out and about (e.g. watching Youtube videos on train journeys) in order to see just how much data I might need. As I approach the end of my first month with Asda Mobile, I see that I'll have used less a half a GB of data for the entire month, meaning that I could switch to paying £10 per month (or probably even just £7 per month) simply by making a phone call to change my regular payment. (There's no contract. I can change the arrangement month-by-month if I wish).
So using my phone certainly isn't mega-expensive.