Plane tickets – Use your favourite flight search engine. If you don't have a favourite, try kiwi.com.
Restaurants – Puglia is not the richest part of Italy, which translates into the local prices. Prices in restaurants are not too high and the food is excellent!
Prices in the destination – As can be expected nice accommodation is not exactly cheap. On the other hand, it is not overpriced either and the choice is huge.
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 – Pretty much any time of year except the winter months (unless you don't mind the colder weather and the occasional rain). In the summer months of July and August, it's very crowded and also very hot here. So it's best to come here in spring or autumn.