'It's the branding, cost of advertising and manufacturers name that usually means the difference in price.'
I think anyone who has bought a cheap cover and found that it wasn't up to the job will disagree with that misleading statement.
We had a Purpleline Wilderness Trail cover - a well known brand. It cost £140 for one to fit our twin axle. It was made from ripstop nylon type material. It didn't last long - the winds pulled it apart at the seams. Purpleline were very good and honoured the 5 year warranty, sending me a brand new one.
I just sold it on and bought a protec cover which is a totally different class of cover. The material is much thicker, breathable and soft on the inside. Specialised are also well recommended.
As for the others - they are the ones that give rise to negative comments about covers. Some people describe them as glorified bin liners. Best avoided.