1

If you are using Mahe is your base - the cheapest method to get to other islands is simply by boat.
An example of cost would be Mahe to Praslin using kat coco boat is 70 Euros return.

Hope this helps, let me know if you need anything more specific.

2

The high-speed catamaran (Cat Cocos) is to Praslin (1 hour) and then you can transfer to La Digue (15 mins). You can see the boat schedule here: http://www.catcocos.com/schedule-2

From what I recall a RT ticket on Cat Cocos from Mahe-Praslin is about 700scr, and the trip from Praslin to La Digue about another 150scr (contact them directly for exact rates).

Your only other option is to fly domestic on Air Seychelles (http://www.airseychelles.com)... I just entered information as if I were going to book a RT flight from Mahe-Praslin and the price was anywhere from $53-$106. I've heard that it often fluctuates so it's best to monitor it and book while the rates are low. They fly back and forth multiple times per day.

Hope this helps!