Model quality and manufacturers.
There isa wide range of model available, from high end models such as CMC and Amalgam. CMC produce diecast metal models and have fully opening panels with superb engine, chassis and interior detailing mainly produced in 1:18 scale (around 20cm) their limited edition models are usually in very high demand. Amalgam produce model in 1:8, 1:12 and 1:18. The 1:18 and 1:12 models are all what is called 'kirbside' which means a sealed resin model with no opening panels. They are CAD scanned and 3D printed for high accuracy, superb paint finish and highly accurate fine scale detailing - such as cromework, windscreen wipers and steering wheels, they also come mounted on a high quality black plinth with a perspex cover.
BBR in Italy produce very good quality models in the same way Amalgam do but are much more reasonably priced, some come with opening panels but are mostly kirbside and come in low number limited edtions.
Other manufacturers such as GP Replicas, Tecnomodel, Autoart, Almost real produce some fabulous models for around £3-400 UK pounds.
At the lower end of the market there are manufacturers like Burago and CMR who make nice models for a low budget. CMR's model are superb value.