If they release costume tgt with hero card which means for those who has the hero, you need to pull again to get the costume version, I think it will be the Hell on Earth in the forum as well. In fact this is why SG gave up on costumed M.Lepus and Killhare, at least for now - so many negative feedbacks from the forum.
For costume only portal, the cost should be much lower and chance is higher to justify it is costume only. If it takes 300 gems for one single pull in the hero portal, I think it should take no more than 100 gem for one single pull in a costume only portal. Pulling 5* costume shall also has a higher chance say at 10-20% (comparable to 15% chance for using ETT during Ninja portal) instead of 2.5%.
If you set the single pull at 300 gems with a 2.5% to get 5*, of course ppl would expect an original hero card to come along with the costume.
Either way, SG needs to find a mechanism to make the system fair to players who have the original hero card or those who don’t. Otherwise there will likely be strong resistance whenever they want to release costumed version for non S1 heroes.