interjection/ˌbalɡame ˈdjos/
dear God! good heavens!
“¡Válgame Dios! ¿El libre se come? ¿Quién podrá tragarse un tomo entero? ¿Que libre es este? ¿Que ha de ser?” — Dear God! You can eat the book? Who would be able to eat an entire volume? What book is this? What is it?
Valga in this context may be an archaism, inherited from an earlier expression in Spanish back when the language still retained some of the many meanings of Latin valere (“to be worth”), one of its meanings being "to be well, to be healthy". Spanish valer (“to be worth”) comes from that word. If so, the expression means "may God give me health" or "may God let me be healthy".