The reason why this has not been implemented yet has to do with the fact that they are not responsible for the prices which are listed here.
As the old (with old I mean the one used until 3 months ago) API didn't support using foil pricing they had to use what they could.
As there is no exact rule on how much a foil cost they did not include a fancy math-part to calculate it by hand card per card.
As the new API was released, i noticed that they had answered somewhere in a post here on the forum that they will implement it in the near future.
However as I am a developer myself, I know that doing this takes far more into consideration then just a little work!
As far as i can tell the team of developers here are doing a great job (as the site still is free, keep that in mind!).
There are some issues here and there, but hey, nobody keeps you from developing a site which has the features that you want to have!
Towards the sellers such as Kaskade I would recommend to ask a feature which informs both people in a trade that there are foils in the trade pool.
This feature is not that long to code and looks like this (in C#)
public bool FoilInTrade(List<Card> userOneCards, List<Card> userTwoCards)
{
var foilsOne = (from card in userOneCards
where card.Foil == true
select card).ToList();
var foilsTwo = (from card in userTwoCards
where card.Foil == true
select card).ToList();
return (foilsOne.Count + foilsTwo.Count) > 0;
}
Anyway I would really suggest that you check out tcgplayer's API guide in order to know what exactly is possible and what not, because promo, textless and other stuff is impossible to use (for now).
Regards,
YouryDW