He's bought some decent players, but I don't think he's used his money well at all. He bought Bennett, Luna, Bacuna and Lowton as full backs, none of whom were any good there, so he's had to go back to using a player that was frozen out for two years. Then there are the nothing players like KEA, Tonev, Bowery, Sylla and so on who individually didn't cost much, but when you add it all up we'd have been much better with fewer, more expensive players,
As everybody rightly points out though, it's his complete lack of coaching ability that is the main reason he should be sacked.
But all of those players probably cost around 10m alltogether. I think the most expensive of the bunch was probably Lowton for about 2m. They were mostly cheap buys in the hopes that they could improve into a decent PL player. You will always get misses when you do that, and even from your list Bacuna, Lowton and KEA were / are ok. They aren't the best but can fill a position in the squad and deputise in the case of injury.
If you wanted to be sure that you would fill those positions with proven quality you'd probably be talking about at least 5m per player.