Well quite a lot of times that increase is purely because we bought them, not because they are actually better.
If you take any cheap lower league/small foreign league player and put them in a premier league team their value is boosted a bit just because they are now a 'Premier League player'.
They are still the same mediocre cheap player you bought though.
Some are mediocre but IMO likes of Guzan, Benteke, Lowton, Okore, Vlaar have been excellent signings which outweigh the bad (likes of Bowery). Jury's still out on likes of Kozak (doesn't look good so far), Helenius, Tonev
I'm not defending Lambert as i'm far from his biggest fan but with the budget he's had I think he's done pretty well and I don't think we're far off from having a decent line up. E.g. Lowton, Okore and Nzogbia (not that I think that'll happen) in for Baker, Weimann and Tonev is a good team.