Leicester I can agree with in some way as they found some good players in past seasons and somehow won the league. They got £10's of millions extra revenue from Champions League and a bit more with global exposure. They also got £200m selling Mahrez, Chilwell, Kante and Maguire in just 4 transfers alone.
I feel they will revert to type in seasons to come and be in amongst the bottom half. I doubt they will be able to substain the money required to stay up there and challenge for Europa League scraps. Leicester have a £162m NET debt and got less revenue than NUFC yet their average wage is £73k compared to your £52k. A difference of £21k weekly is over £1m more per season per person, if both had similar numbers of personel. Look at the 4 images and the rise in their wages cost as from their 1st season back in the PL in 2015, it's gone from £57m to £157m in 2020 and it's doubled since they won the league.
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Looking at the tweet above and the big wage spending chasing success reminds me of the Mags in the 90's, along with Leeds and Blackburn. Aye they're up there now but long term it's unlikely the money will keep coming in like it has been for them to maintain (well push it to the limits given it's over 100%) that spending level. Also Schmeichel and Vardy (lacking goals even now) won't be around forever and past their prime.
Southampton have only been out the top league 7 seasons since 1977-78 and a total of 11 seasons since 1967-68 so they're were they've been for the last 50 odd years. However for all the time in the top league, they've finished top 10 a total of 13 times since 1968 in these positions and 4 of those since coming back up in 2013 as shown in brackets.
2nd - 1
5th - 1
6th - 2 (1)
7th - 5 (1)
8th - 4 (2)
They're no denying they had a decent spell a for 4 seasons after coming back up (is that due to the extra money) but they keep selling the good good players and they got £110m from Van Dyke/Mane alone. Prior to that they also sold Shaw/Llalana for £60m along with £100m from Lovren, Chambers, Clyne in those 4 seasons. They'll probably cash in on James Ward Prowse if he has a decent season. Even last season sold 2 they should really be keeping in Ings/Vestergaard. They are reverting to type in the last 4 seasons in the lower half.
The thing is, going down to League One for them was also a reset in some way to clear out the higher expenses like we have done. Southampton dropped to League One but then got promoted to the Championship and straight into the big money Premier League. They got the extra money, had 4 good seasons selling players for big money but the last 4 seasons haven't been as good and they lost £76m in 2020 finances and had a higher wage bill than NUFC.
So Leicester and Southampton may have invested more but they've made some good money selling players. Of the 3, I'd put Leicester first with the Mags in 2nd place and Southampton 3rd as they're closer to where you'd expect. The big question is will Leicester continue to prosper or has their 'success' bubble started to deflate? Will they be able to keep paying high wages and invest like they have if they start to drop down the table? I only put Leicester top for now and that's more to do with them being able to find decent players to also possibly sell have clubs ever been able to do this consistently?
This was just out of interest more than anything as I've just google most of the speil I've just spouted. If anything it shows me that there's more to look at than just NET spend or the other limited things most football fans may focus on. Leicester may look pretty on the outside at the moment but long term who knows what could happen if they end up back in the lower high with such high costs.