Why are some GoldStar cards so much more rare and expensive

Latias, latios and Rayquaza are from a set (EX Deoxys) that is one of the rarest sets in the tcg. It is all about supply and demand.

Ok, that’s a bit discouraging. I guess the same is true for the Gen3 starters?

Yes

Mostly Torchic due to pop; Treecko and Mudkip been selling for around U$S1000 or even less during last few sales; torchic in the other hand is going for way more due to the low pop (Pop15 card): Last 2 confirmed sales went for over U$S4500