What is a copy of violin marked Antonius Stradivarius facibat cremona 1713 by Salvadore de Durro worth?

It is impossible to provide an accurate estimate of the value of a copy of a violin made by Salvadore de Durro without examining the instrument in person. The actual value of a violin depends on several factors, including its history, craftsmanship, overall condition, rarity, and demand.

For an accurate evaluation, I recommend hiring a professional violin appraiser experienced in assessing fine string instruments. They can examine the violin's craftsmanship, materials used, provenance, and other vital factors to give you a reliable estimation of its market value.