Costar un pastón

Costar un pastón
 

It means costs an arm and a leg. I would like to know if it is international
or Spain-specific.

I believe it is Spain specific. But it is not spelled "postón" but "pastón"

It comes from the word "pasta" (dough) meaning money, a lot of money... changed with an augmentative (pertaining to or productive of a form denoting increased size or intensity. In Spanish the augmentative suffix -ón is added to a word (as silla “chair”) to indicate increased size, as in sillón “armchair.”.)

I have also heard "costar un porrón [de dinero]"

Title corrected.

It wouldn't be understood in Mexico. We might say an equivalent as "costar una lana" or "costar un lanal" or "costar una lanota". :)

From 39 instances in CREA for "pastón", 36 are from Spain and 3 from Argentina (where "pastón" means a couple of things: one meaning related to art and fashion, the other one related to masonry).

I suspect "pastón" is more like "a load of money" and not "an awful lot of money" as "an arm and a leg" ("un ojo de la cara") may suggest.

El diccionario de argot de Espasa (Julia Sanmartín Sáenz) da, pastón mucho dinero. Le tocó un pastón en la lotería de Navidad.

Yo lo he oído y visto en España equivalente a:
Cuesta "un ojo de la cara" "Un riñón... y parte del otro"

Hay ejemplos a montones, como este que copio
... creo que es una guitarra ideal, y tampoco cuesta un pastón, cuesta lo suyo, pero tampoco como las js nuevas que cuestan más de 5.000 euracos.

(Me encanta los de los "euracos", rima con "atracos")