This is sort of a more detailed version of @AnonymousPhysicist's answer, specifically for the UK.
In the UK all academics (with the exception of Imperial College London) are paid on the same 54 point national pay spine. The salary at each point is negotiated by the unions and a organization that represents employers each year. I'd not say that the unions are "powerful" as they have lost this negotiation every year for the last 10 years, and pay increases have been zero or below inflation.
Each university can divide those points up into "grades" however they like, but most are more or less similar in practice. At my uni we have 9 grades. What grade you are is determined by your job description, not your subject area. So grade 8 is described as "having principle responsibility for delivering one or more areas of teaching and leading a program of research, either of which may involve management of junior colleagues". The complete job descriptions for each grade are available publicly on the internet. Grade 8 starts at point 37, which in 2019 was £41,526 GBP ($57,667 USD). All academics taking up an entry level faculty position will be grade 8, the vast majority grade 8.1 (sometimes people negotiate to start at 8.2 or 8.3), irrespective of their discipline.
This is useful for universities on some level - tuition fees at the undergraduate level are capped at £9,500 for home students by law, but some degrees (principally experimental science and engineering courses) cost way more than this to teach. Degrees that are either cheaper to teach (like humanities) or can attract significant overseas students (like engineer) cross subsidies expensive subjects that can do neither. MBAs are both cheap to teach and attract many high-fee paying overseas students, and so provide a large amount of cross-subsidies to, say Biology, which costs twice the fee cap to teach, but attracts few overseas students.