Some more data and analysis to share. Looks like the mean PQD spend for GS qualifiers is in the range of $56-61K. Since last year, the average PQD has risen by about $4K USD. See below:
Year: Mean PQD Spend (for GS Qualifiers only)
2015: $56,160
2016: $60,128
2017: $57,521
2018: $61,543
So based on the above, it appears that there is no absolute minimum spend. Since the mean PQD varies from year to year, UA is probably skimming off a fixed percentage of flyers from the top of their pool.
Some have postulated that a spend of $50K would guarantee GS status. There are many cases of people spending $50K+ in the spreadsheets who do not get invited to GS. It appears to be driven by high PQD spend, as well as most of that being on UA metal. The 3 year running average is important only after getting your first GS invite.
Links to the last 4 years of data collection are provided below: