Plane tickets – Use your favourite flight search engine. If you don't have a favourite, try kiwi.com.
Restaurants – Prices are a bit higher. They are usually around 20 EUR for a main course. Many restaurants are closed during low season.
Prices at the destination – Generally, prices are quite high. For example, a simple fridge magnet (the traditional souvenir of every tourist) normally costs 3-5 EUR here. Getting a coffee for 1.5 EUR is exceptionally rare, most commonly it costs between 2 and 3 EUR.
Calls and data – For EU citizens, everything is the same as at home. WiFi is common in hotels and restaurants and is free.
When to go – This is a major decision. By the end of March and April the weather will be nice and there won't be as many tourists, but the lavender won't be in bloom yet. On the other hand, at the end of June and the beginning of July, when the lavenders are in bloom, there will be an incredible number of people. We were here at the end of March, the daily temperatures were between 15 and 22°C, and there were practically no tourists (which we really appreciated).