I know a little bit about the Spey but very little about the Dee.
The best time to fish the Spey is now or July. Now the fish are in their prime spring run. July it is mainly grilse, but they are more plentiful, but smaller.
Costs for the best beats Tulcan etc £1000/ week + accomadation. $--- £1.4 x roughly.
Tulcan have their own web page http://www.tulchan.com/