Looking at how much both managers have spent, without considering what they have sold over that period provides no context and is misleading. Would Liverpool have purchased the new players they have this season had they not sold off existing players to fund the investment?
Liverpool
In the 16/17 season, Liverpool made a transfer market profit of GBP6.5m (Transfermarket). In 17/18, we made a GBP28m profit (Does not include Keita purchase, as he'll only start for Liverpool in the upcoming season. This profit is obviously driven by the sale of Coutinho. For 18/19, we've bought Keita for 52m, Fabinho for 43m and hopefully Fekir for another 53m or so. Circa 148m spend this season, but considering the previous two seasons have resulted in net transfer surplus's, the overall investment over Klopp's tenure to date is hardly outrageous.
Klopp's net spend since summer Summer 16/17 = GBP113.5m
Man United
In the 16/17 season, Man Utd had a transfer net spend of GBP102m (Transfermarket). In 17/18, that net spend was GBP136m. Add to that the two transfers made so far (Fred for Circa 52m and Dalot for 19m) and you have a net transfer spend of GBP309m.
I'm not sure it could be objectively argued on this basis that Klopp has invested anything like what Mourinho has in their respective squads, over the same timeframe. His net spend on the Liverpool squad is less than 37% of United's net spend (assuming Transfermarket figures are reasonable accurate).