That is an idiom of the english language which doesn't have a direct translation. In figurative sense it is used to thank, being really grateful towards another person. A similar or equivalent phrase in spanish would be: "¡Muchísimas gracias!" Normally when you want to thank someone you would say: ¡Gracias! (Thanks!) or ¡Muchas gracias! (Thanks a lot!), but in this case, the superlative "-ísimo" is added to "Muchas", increasing or exaggerating the meaning, resulting in the equivalent expression: "Thanks a billion!"

Yo suelo decir (con el mismo significado que el de tu frase) MIL GRACIAS tal y como apuntaba Jarib.

"Mil gracias" is also quite common.

You can say: "Un millón de gracias" It's quite common.

Te agradezco mucho, te agradezco un montón, muchísimas gracias.