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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Even cheaper and more modular (since it is free as in freedom and open source DIY) is the Relativty headset, which can be built for only $200 USD. I also highly recommend considering this option. Although it probably won't get as good tracking as the Vive, it has a higher resolution and refresh rate and is even more modular due to being built from off-the-shelf parts and fully free and open source.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.