Many of the cars have improved greatly from the first releases; Sector 3 are getting the real world drivers to test and fettle the physics and it's not just marketing bullshit. Many of the series licence holders actually demand their drivers test what Sector 3 have made. That means the DTM, GT3, ADAC, Audi TT RS & WTCC have been tested by at least one real world driver and are as close to the real thing as Sector 3 and/or a commerical grade sim can get.
That said, physics realism and tyre modelling is so subjective it's laughable how many waffle on about Sim A or Sim B being 'the best' or 'most realistic'. How does one judge when one hasn't sat in, let alone driven said vehicle? All I know is the area is almost impossible to get close to 100% realism and very difficult to make something that is about 85% of the real thing. If a sim uses BoP for multiplayer/AI, many aspects of realism are fucked up anyway
Costs are determined by RaceRoom and not Sector 3 - it's expensive because you won't get fully licenced cars, tracks & team liveries anywhere else on PC.
Despite what Sector 3 and SMS will say,
there are a few guys still at Sector 3 that were a part of the SimBin+Blimey! Games teams that created GTR, GTR 2 and GT Legends. The same can be said for SMS too. Who made what and how for these sims will always be a battle between studio heads.
I know a few GTR 2 dev's have headed to the team making the Isle of Mann TT and WRC 5.