I’d better put it in Spanish too: Como enviar una carta desde España a Suecia usando sellos “Tarifa A”.
Spanish “Tarifa A” stamps don’t have a printed value. Instead their value is whatever the cost is to send a normal 20g letter within the Iberian peninsula. So in order to send a letter, one has to perform a three-step process at www.correos.es:

• Click “Enviar Documentos” and then “Calculador de tarifas” and select “Enviar Documentos”.  In the form that loads,
  • select “Carta y tarjeta postal internacional”, enter the weight (10g in my case),
  • do NOT tick “Normalizado” after doing some math: a C5 envelope is made for an A4 sheet (which I happen to know is 210×297 mm) folded in half, which should give 210×149, which does not fit in either “14×9 cm. y 23,5×12 cm”
  • select Destino: Suecia
  • wonder why there are two places where you pick origin, and leave both on “Península y Baleares”
  • click “Obtenga su tarifa”
  • write down “1,55″ somewhere and assume that it is Euros.
• Enter “cuanto vale un sello tarifa A” in the search box.  This gives the helpful answer “El valor de un sello con la tarifa A, es el de la tarifa de una carta con origen y destino dentro de España, cuyo peso sea igual o inferior a 20 gramos de peso y sus dimensiones sean normalizadas”.
• Find your way back to the calculator, and this time pick
  • Carta y tarjeta postal nacional
  • 20 g
  • “Normalizado”
  • Click, tadaa: 0,45.

Now we have all the data and all that is left is the calculation: 1,55/0,45 = 3,444444444, so no less than 4 stamps.  OK.

Redoing the calculation for a “normal” (C65?) envelope gives a price of 1.15 which is 2,555555556 stamps.

Update 2016-03-11: Finding the value of an A2 stamp is nigh impossible, but my best guess is that it’s worth 0,57€.  The guess is based on an article published by Correos Gallego.