Oftentimes it's not about the cap number, but rather the player.
Take the running back position. Recent history suggests teams should invest in backs via the draft because there are productive players available in the middle rounds at a fraction of the cost of a free agent running back. Le'Veon Bell, a former second-round pick, is on a four-year, $4.1 million deal. Donald Brown signed a three-year, $10.5 million deal in free agency. There are endless examples. We won't waste time listing them.
Point is, for most teams, paying a free agent running back is not practical.