I know this may not be very accurate but here is what transfermarkt.co.uk values the top 10 central defenders and top 10 left backs:
Center backs in Euros:
1. Pique 38m
2. Vidic 37m
3. Terry 30m
4. Thiago Silva 30m
5. Pepe (RM) 28m
6. Chiellini 26m
7. Kompany 25m
8. Ivanovic 23m
9. Luiz 23m
10. Hummels (Dortmund)18m
11. Subotic (Dortmund) 18m
12. Ranocchia 17.5m
13. Sakho (PSG) 16m
14. Toure (Mancity) 15m
15. Lescott (Mancity) 15m
Bruno Alves is 20th and valued at 14m
Now Left backs:
1. Lahm 28m
2. Cole 25m
3. Evra 25m
4. Marcelo (RM) 25m
5. Coentrao (RM) 24m
6. Kolarov 15m
7. Pereira (Porto) 15m
8. Clichy 14m
9. Abidal 13m
10. Criscito 12.5m
11. Adriano (Barca) 12m
12. Ansaldi (Rubin) 12m
13. Cissokho (Lyon) 10m
14. Assou-Ekotto (Tottenham) 9.5m
15. Mathieu (Valencia) 9m
Bottom line, if we wanted to splurge on left back and center back its hard to see how we would spend more than EU30-35m. Krasic and Elia could net us 20-25m.
Problem is, if Kolarov is not that good, its hard to see who is a great left back out there that would want to join us. The best one is Lahm or Evra but he would never leave Bayern & Man U.
Dream would be Hummels and Evra but thats fantasy football.