You're not wrong, but Nimmo will certainly be more than just "$6-8 million more" given how much longer of a commitment he'll require to sign, and upside wise if anything Bellinger theoretically provides more, but Nimmo for sure offers that present certainty. It's much more difficult to answer the question of
a) 1-year reclamation project at <$15M, could be anywhere from a 1-6 WAR player, projects at ~1.5-2 wins. CF defense + good baserunning more or less guaranteed, offense can swing the whole value
5+ year commitment on guy who will be 30 at the beginning of the season, $100M+ committed, 3-5 WAR depending on health in 2023. Defense is "ok" at CF, likely will decline within a couple of years, health a question mark, baserunning exactly average, offense likely to be excellent for a couple of years.