Hi Max.
I've only done Piran as an included excursion on the Slovenia trip a couple of years ago. JY have adapted that trip and it is now called From the Alps to the Adriatic, and it is still included on that tour. So I got a little confused. :)
I can see that this is an optional excursion on the Lake Bled Christmas trip? It does seem a little steep but it is a fair journey time wise from Bled down to the coast and if lunch is included I would say that is a fair price. Piran itself is only small but a lovely little place. Beautiful buildings and the coastline is really nice. If you get a good day weather wise then I would say it is certainly worthwhile, but of course you cannot guarantee that. Weirdly enough the time we had in Piran was the only day that we didn't get brilliant sunshine the whole week. The journey down is very nice with some beautiful scenery. If you want to see something of the Slovenian coast then I would certainly recommend it.