The words 'lui' and 'en' are both pronouns referring back to nouns mentioned in the previous sentence. The pronoun 'lui' refers back to the woman who bought the car, and the pronoun 'en' refers back to 'euros'. Note that at the end of the second sentence there is just a figure, but no repetition of the word 'euros'. You could translate the phrase as: " ..... would have cost her more than 25,000 of them." The 'lui' is 'her' (the woman) and the 'en' is 'of them' (euros). I hope that helps.

Ici on utilise la forme impersonnelle: il lui en coûte = cela lui coûte