Nothing special to buy in Visegrad but next to castle entrance there are numerous small souvenir stores selling the usual items like magnets, tshirts, cups in medieval like shapes etc
There are also some jewelry stores but also some with local products, liquers and other drinks.
This market gives you opportunity to buy some gifts, from visegrad and Hungary. There are many national thing in sale there like pencils with national ornaments, clothes, postcards and more goods.
About postcards: I am happy I have bought the postcards with Visegrad in this place, because I havent found it in Budapest. One more thing - postards are cheaper in little shop inside the castle.
What to buy:
pencils with national ornaments
and more goods.
What to pay:
It depends on that you want to buy. For example, postcard cost from 50 to 100 forints (about 0,25 euro).