Free, gratis and at no cost. But potentially very valuable to you!
Kyle Martin posted a little while ago about a free tool that will be of great use to anyone who needs to compare to versions of a model in Revit.
The Model Compare tool for Revit is currently only available for 2017 -2014 whereas this one works in Revit 2018.
This is not a new tool but it is well worth taking the short time you’ll need to download it and check it out. Cool stuff.
AEC Technology Hackathon 2016 — Kyle C Martin
Metamorphosis | Devpost