Interesting claim, but if you use one of the Draft Value Charts as a proxy, there's as much value in the picks 51+ as there is in picks 1-8, i.e. the sum of draft value of picks 1-8 is 15800, and 51 to the last pick totals 15218. It takes 206 picks to equal the value of the 8 top picks, but the draft value chart says those 206 picks aggregated have roughly the same expected value as the top 8 aggregated.
But if you go to the top 50, total draft value is 45805 or 3X the value of 51+. So if you built a team out of the top 50 in this years draft, you'd need to take all 600+ players from 3 years of drafting 51+ to equal the total value.
So here's a version of your statement based on this analysis:
3/4 of the value in a draft is gone by pick 50.
2020 NFL Trade Value Chart
I used that chart because it is in a format that's easy to load into a spreadsheet.
You can also proxy Free Agent value too: late 7th round picks are assigned 1 pt of value. So if each team signs 10 free agents, that's at most 320 points of value, or the equivalent of the 26th pick in the 2nd round -- for the entire class of free agents.