£106.2 million net over the last 5 years according to here:
https://www.transfermarkt.co.uk/premier-league/fuenfjahresvergleich/wettbewerb/GB1
Note however that this includes January under Mourinho so you can remove the £33 million or so of outgoings for Bergwjin and Gedson's loan fee and the £20 odd million of income from Eriksen's departure and Rose and KWP's loan fees.
So Pochettino actually spent about £93 million net over 5 years. To put that into context of how it compares to the other clubs currently in the PL, this is the 15th highest net spend in the PL over that period. Only Norwich, Sheffield United, Burnley, Southampton and Crystal Palace have spent less than us in that same time period. If you want to add the Championship to that as well then we slip even lower with Fulham and Stoke having spent more than us over that same 5 year period. a sobering thought really. Even more sobering is the fact that if you ignore last summer's signings that Pochettino got virtually no chance to work with at all, and take just the four year period before that then it is only Norwich, Southampton and Sheffield United who spent less than Pochettino's Spurs, of course only Southampton were PL ever presents.
I would imagine the windows when Pochettino didn't get his targets and made do with cheap signings instead may have caused him to turn down sub-standard players in later windows. Of course we know that he definitely wanted Grealish only for the chairman to balls up a deal that could've been done for a very reasonable amount (I expect Grealish will now move clubs this summer for around £50 million, when he could've spent a year understudying Eriksen and smoothly replaced him this season).
If I look at those highly rated youth players that didn't kick on, none of them have gone on to anything significant in their career. I think that probably suggests that they simply weren't good enough players or had the wrong attitude to make it to the top?
Re: Bentaleb and Mason, I think Pochettino just upgraded them with the likes of Dier and Wanyama. For every good prospect signing that didn't progress under him, there was one that clearly improved under him.... (Dele, Trippier, Dier, Son, Wimmer, Davies) That is the nature of 'prospects' you are going to have a patchy hit rate as you are buying potential instead of proven players.
A couple of those big ticket signings didn't even really get a chance to play under Pochettino, Sessegnon arrived injured (Remember that Poch also wanted to bring him in a year earlier, I guess to spend a year getting up to speed so that they could release Rose in Summer 19, i.e. succession planning, like with Eriksen and Grealish). Also Ndombele started really well for Pochettino before getting injured, he looked far better at the start of the season than he does now under Mourinho, where it seems as though he doesn't want to play for the team at all. So that leaves Sanchez and Sissoko. I think Sanchez did alright under Pochettino, not brilliant, but OK for a young centre half. Sissoko also came good after a terrible first season and an improving second season, does Sissoko's improvement not perhaps evidence some good coaching from Pochettino?
I expect Pochettino to do brilliantly at the next club he manages, especially if the chairman listens to him and buys and sells the players he wants and properly backs him with money for signings instead of giving him less to spend than three quarters of the clubs in the same league.