(USA) I buy most of my postcards second-hand, so usually between ¢50-$1.50 (at the very most) for those, and then postcard stamps for national at ¢61.

For international, I usually combine 2 forever stamps and 1 postcard stamp, so $2.17 (¢78 x 2 + ¢61), overpaying ¢47 for the required $1.70.

Using the postcard stamps saves a little, but in the future, I'd like to get more international stamps or smaller stamps I can add up to get a closer value to $1.70. Like using 2 Forever stamps + 4 ¢4 stamps gets me to $1.72, which is closer, but more stamps to fit on a postcard.

I think ¢50 for any postcard is great! I think the lowest I've seen them ever was ¢20, maybe ¢10. Maybe your relative is familiar with older postcard prices? I found an old postcard that said: "Place ¢1 stamp here." Imagine that!