

I’d think the opportunity cost is easier to realise with the absolute difference in currency units. The price of any alternatives you might consider is usually specified in currency units.
The only relative that I’d think might be helpful for expensive purchases would be to convert a price orrice difference into number of days/ week / months of your expected disposable income or something like that.
I use CDs, Records and occasionally bandcamp - usually just for a free listen before I go to their concerts and buy from them there.
But just to be clear I also buy a lot of used records and cds; which I think can be seen as similarly immoral. Evil used record stores hoarding all the margin and never compensating the original artists. Again if they’re any good and ever play nearby, they’ll get a ticket sale and maybe I’ll by a record direct from them if they do a good show.
Mostly I only care about the morality for small/new/unpopular bands, first few albums and so on.
I couldn’t give a shit about compensating large successful artists like the Rolling Stones or Bob Dylan - or more accurately, whoever they sold their catalogue rights to.