I often see people say to buy the expensive toilet roll but I always go for the low end of the midrange rolls. I don't need 4ply, scented, quilted shit, I just need two pieces of paper stuck together.
There is a balance, you don't want to get the 1ply stuff you'd find in a stingy office. But just look at the label and price-per-sheet
(If you can afford it, buy a year's supply in bulk from the company)
Unfortunately I haven't been able to find a bidet attachment that fits my toilet. When we move, I'm getting one for sure but I'll still use toilet roll at times too
That being said, I'm not sure the cost-benefit really fits here. The initial cost (£100-500, depending on quality and type) plus fitting (£100-200 depending on plumbers in your area) would take about 10 years to break even when spending £40 on toilet roll per year. And by then, I'm sure I would have moved house or the bidet would have broken or something