Of course his cost matters, as well as his salary. It is all part of how well (or poorly) a club is run. His huge wages are why you'll have trouble moving him on (if that's what ETH wants), and you'll have to take a significant loss with the transfer fee. So far, he has not turned out to be a good investment. That may change, but at the moment I think that's the reality. As
GifLord says, you are paying star wages to a player whose output hasn't matched that, and has looked more like a squad player. He is your 2nd highest earner (after De Gea) yet he's not a guaranteed starter a lot of the time.
Do you realise that United have spent about £1 billion since SAF retired? The return on that has been 4 trophies - FA Cup, Carabao/League Cup x2, and EL. And perhaps only one serious title challenge during that time, and finishing outside top 4 half the time. Would you say that your ROI on £1 billion has been good? You spent £80mil on what's turned out to be a backup CB, is that not ridiculous?
I used to not get caught up in transfers and salaries too, but Arsenal have been in a similar position to you guys and we also had to overhaul our squad. We had to get rid of problematic stars (Ozil, Auba whose wages were similar to Sancho's) and had a disjointed squad before Arteta arrived. We were paying very high wages despite not being in Europe. We have since reformed our transfer policy and restructured our wage bill so hopefully don't get caught in those situations again. We also manage player contracts better by not letting them run down. Since Arteta arrived, I think he's been the highest spender in his 3 years, spending about £350mil. Though Chelsea's recent spree in Jan has probably changed that. But this investment (which continues) is necessary because we had a poor squad which was 8th when he arrived and not being in CL for many years has massively hurt our revenue. You can bet that with how hard Arteta has been backed, he will be expected to deliver trophies, title challenges and at the very least stay in CL for the next few years. Anything less would be a failure considering what he's spent.