Shouldn't really need explaining. Calciopoli, that thing that robbed us of all but 4-5 of our best players. And they were older players at that, with only Buffon currently remaining.
If I take Bayern transfer spending vs Juve's overall since the 2007-08 season (when Juve returned to Serie A and had to rebuild a team) then the net spend is almost the same, just over €200m. Juve have spent more overall, but you balance it out net spend because sometimes you sell to buy, like Milan.
This is Bayern who have been in the CL proper for all but one of the last 16 seasons.
Post calciopoli period spendings:
Transfer expenditures: 47.432.000 £ Juventus 12/13
Transfer expenditures: 61.864.000 £ Bayern 12/13
Transfer expenditures: 88.616.000 £ Juventus 11/12
Transfer expenditures: 38.808.000 £ Bayern 11/12
Transfer expenditures: 53.310.400 £ Juventus 10/11
Transfer expenditures: 14.960.000 £ Bayern 10/11
Transfer expenditures: 52.184.000 £ Juventus 09/10
Transfer expenditures: 65.736.000 £ Bayern 09/10
Transfer expenditures: 35.596.000 £ Juventus 08/09
Transfer expenditures: 0 £ Bayern 08/09
Transfer expenditures: 63.756.000 £ Juventus 07/08
Transfer expenditures: 77.616.000 £ Bayern 07/08
Transfer expenditures: 3.520.000 £ Juventus 06/07
Transfer expenditures: 21.120.000 £ Bayern 06/07
Total Bayern: 280.104.000 £
Total Juventus: 344.414.000 £