Which beat are you on on the Dee? March should be good especially if paying £100/day is that for fishing and accomadation or just the fishing?
At the backend, Autumn, the Dee and the Spey both close on the 30th September, the Ness system and the Beauly both continue till the 15th October and the Deveron and Don finish the 31st October. From then on you have to go down to the borders where the Tweed system runs till the end of November as does some of the others. Tweed can be expensive up to £1000/day.
Association, club fishing is cheaper,consider Grantown on Spey for early season April.May and the Ness town water for October, remember though these town waters can be busy especially on Saturdays.