A six year contract and a £50m transfer fee for Kelvin Phillips on roughly his current salary would be around £65m. A six year contract and £50m transfer fee for Rashford on his current salary would be around £125m.
Not only that but Rashford's wages prevent 90% of clubs that could afford a £50m transfer fee, such as West Ham, from buying him due to being unable to break their wage structure (so even if they could afford a £125m package they couldn't deal with the consequences on the rest of their squad). Kelvin Phillips on the other hand even at a £50m pricetag would be attractive to a couple of dozen clubs.
Transfer fees are irrelevant when not taking wages into consideration.