Just to supplement the information provided by #1 and #2:
A price of 300+ euros for a one-way flight Tana to Sambava suggests to me a routing via Antsiranana (Diego Suarez) into the weekly direct flight from Diego down to Sambava. The prices for the two one-way flights add up to more than 300 euros. Air Mad have a bad habit on their domestic services of simply adding together the segment prices instead of making allowance for the maximum permitted mileage.
The price on the Air Mad website for a direct flight Tana – Sambava is as cited by TexasTea at #1.
At present, it is always cheaper to book domestic (and if you can work it international) flights in Madagascar at an Air Mad office (not at a travel agency licensed to sell their tickets). In September, I paid around 150 euros without the discount you get for flying Air Mad into the country, and around 115 euros with the discount for a one-way domestic flight selling at 199 euros on the website.
If you think you may not have time to book when you arrive or are afraid that your chosen flight will be booked up, you should consider flying into Madagascar with Air Mad, either from Paris or from one of the African or Indian Ocean points served by them. You’ll often find that even if your international flight is cheaper with another airline, once you add in the cost of one or two domestic flights Air Mad works out cheaper.