Visigothic coinage was mainly based on gold. Its principle denomination was the tremissis, which corresponds to a third of a solidus. These small gold coin (they weighted 1.52 grams) were massively produced in Hispania and Gaul during the end of the fifth and the year 711, when Visigothic kingdom collapsed due to the Arab invasion.
Tulga was a Visigothic king from 640 to 642. During his short reign he minted coins in Sevilla, Córdoba, Zaragoza, Mérida, Narbona, Idahna a Velha, Granada and Lugo. So many mints for such a short reign makes all these coins to be rare and some of them to be extremely rare.
Visigothic coinage is not on fashion nowadays. Hammer prices are not as high as they used to and many of the common coins are not sold when they appear in auctions. While it can be seen as bad news for some collectors, many others will understand it as an opportunity to find rare coins for reasonable prices.