Porqué Imperfecto? No es algo habitual como: Siempre me costaba...

Pues, se podría decir: En aquel entonces te costaba...

Pero creo que el original aquí quiere más bien expresar una acción individual.

It’s just a little bit confusing having the very same sentence in two tenses.
We should rather ask the original poster what he wanted to express in the first place...

Sentence #7120359 is a duplicate, but a much better formulation.

Tanto "costó" como "costaba" se corresponden al inglés "cost".

Si usas "costó" significa claramente que el objeto fue comprado y hubo que pagar mucho por él. ➜ Esta bici me costó un ojo de la cara.

Si dices "costaba", puede significar dos cosas:

1) que algo era muy caro en cierto momento (por ejemplo, hace mucho tiempo) ➜ Cuando era pequeño, volar en avión costaba un ojo de la cara pero ahora está tirado de precio.

2) que el precio era una circunstancia (en este caso negativa) ➜ No me compré la bufanda porque costaba un ojo de la cara.

Of course, but you are essentially just repeating the two examples I gave above.

My point is that formally it is true that the English Past Tense can correspond to the Spanish Indefinido or Imperfecto, but logically the speaker of the utterance has only one situation in mind.

If the example sentence contained certain characteristic (contextual) components it would be clear which tense to use in Spanish.

But as far as I can tell it is beyond the idea of Tatoeba to avoid ambiguity.

In this case you would always have to translate every even simplest of sentence into two or even more versions.

What about ‚hoy me he comprado esta bici‘? Same catch! Some regions prefer to use the Indefinido here despite the very clear rules of the distinction between Perfecto and Indefinido.

This fact renders almost all examples in Past Tenses useless for practicing...

Anyways, thanks for trying to help ;-)))

In this case context could be provided for a previous sentence uttered by somebody else:

* Case 1) ¿Volar en los 80 era tan caro como ahora? / Costaba un ojo de la cara.
* Case 2) ¿Por que no te has comprado la bufanda? / Costaba (o cuesta) un ojo de la cara.

But, as you say that's a bit beyond the Tatoeba ways... although I have come across similar "dialogues" here more than once.

Regards.