I buy my stuff from JustVape247. I don't think I can post links yet (?) but the address is typed on the big bottles in my pic or is their name followed by .com . I've only ever had a problem with them once, where they forgot a few items. I just emailed them and they sorted it out super quickly, so I'd definitely recommend them No idea how competitive their prices are now days, but they definitely were when I started using them a couple of years ago.
Cost-wise hmmm.... Well the 2 bottles (I think 1L each) of VG / PG cost around £7 and the nicotine (100ml) cost £15. I think that easily lasts me 6 - 8 months. The flavourings themselves are like £2 - £3 each, and I think I get about a months worth out of each.
Sorry I'm being vague as I don't keep proper track, but with those figures I guess around £7 per month? To give you context, I'm quite a heavy "vaper", using it at work and at home. I couldn't tell you how many ml I vape a day as I do partial refills / top ups, but do like my juice to be relatively weak. It's definitely a big saving over buying premade juice though, and more fun. Saying that I'm super lazy these days so don't bother mixing flavours
Have you heard of "Planet of the Vapes" by the way? They have user-submitted recipes if you do take the plunge.
Judging by the Kanger Subox Mini Kit you linked, I take it you recoil yourself?