Why would it be that cheap? Fusion 360 is competing with SolidWorks. A standalone license for SolidWorks probably starts at $4000.
For any comparable commercial 3D parametric mechanical CAD system just the B-rep modeling kernel alone represents several hundred person years of commercial development going back 30 to 40 years, and academic research going back 50 years.
The Fusion 360 kernel is ShapeManager [0], forked from ACIS 7.0 in November 2001.
Work on ACIS [1] started in 1985 after the 3 founders (Ian Braid, Charles Lang, Alan Grayer) left ShapeData having worked on Romulus.
Romulus [2] was released in 1978 and was based on BUILD 2.
BUILD 2 and before that BUILD were created by Ian Braid during his PhD thesis started in 1969 and supervised by Charles Lang. [3]
Now, what about SolidWorks?
The SolidWorks kernel is Parasolid [4], dating from 1987, which was a C dialect rewrite of Romulus, which was written in Fortran. ACIS, by the way, was written in C++, so was one of the first commercial systems written in C++.
How do I know the above? I worked on Parasolid 1989 to 1995. We had 20+ devs then - department head count was 40+. The still have lots of devs working 30 years later.
There's only one comparable open-source B-rep modeling kernel - Open CASCADE, that powers FreeCAD. But quality wise, IMO comparing it with Parasolid, ACIS or ShapeManager is a bit like comparing Lucene with Google or Bing.
CGAL that powers OpenSCAD is high quality - but it's a very different beast, and not something you could use to power Fusion etc.
Why is B-rep modeling so much work? Here are some reasons:
* B-rep models are inherently complicated graph based data-structures with many different node types (geometry and topology). Just keeping the model consistent is a lot of work.
* Lots of surface and curve types means lots of combinations for surface X surface, surface X curve, curve X curve intersection/distance/... algorithms.
* Tolerant modeling.
* Topology case analysis from hell for blending.
* Hybrid b-rep modeling with other approaches, meshes, volumetric etc. and make it work properly.
* Feature-based history modeling layered on top of b-rep modeling means that your b-rep operations must replay EXACTLY the same, even when you upgrade the kernel.
[0] https://en.wikipedia.org/wiki/ShapeManager
[1] https://en.wikipedia.org/wiki/ACIS
[2] https://en.wikipedia.org/wiki/Romulus_(b-rep_solid_modeler)
[3] http://solidmodeling.org/awards/bezier-award/i-braid-a-graye...