I don't understand why Redcafe is so obsessed with transfer spending.
50% of transfer don't work (detailed research study by Paul Tomkins and a few others, in Pay As You Play book). That figure applies as much to expensive, glamour signings, as it does to value for money transfers.
Since Sir Alex retired, the club has spent a fortune on transfers. This forum (as a whole) was wildly enthusiastic about the signings of: Mata, Sanchez, Schweinsteiger, Di Maria, Falcao, Martial, Shaw, Zlatan, Pogba, Mkhitaryan, Memphis, Matic. The view on the signings of Blind, Herrera, Schneiderlin, Bailly, Lindelof, Rojo, Fred, Dalot was positive, though not as much excitement. I can't recall what the overall opinion of Darmian, Romero or Lukaku was. The only signing that the forum was against was Fellaini.
Some of those signings have been a success, some didn't make much difference, a few were a total flop. But if you look at how many made an impact in year 1, the number of clear successes is very small. Part of the reason for transfers is preparing for the future, rather than immediate contribution.
If you want to figure out what is realistic. Take the number of points we get in the 2nd half of the season, then double it. You can then adjust according to how many players you believe will perform better next season than they did this, and how many are in decline. Also whether Solskjaer's coaching team can resolve long standing issues, such as our build-up play from the back, right-side of the pitch, conceding fewer big chances, and final third creativity.
Currently we have 32 points from 14 games in the 2nd half of the season. If we were to beat Cardiff, Huddersfield and at least one other team, that would take us above 40 points. The team that finishes 3rd this season, will probably have slightly less than 80 points; the only way to beat that total is for a team to win all its remaining fixtures.
Or you could believe that spending £300m will make a big difference, £200m some improvement, £100m no impact at all.