The ability itself usually has a cost. Incandescent Soulstoke requires 1R and tapping him. Elvish Piper, requires G and tapping him. THAT cost youmust pay. Also, when you "put something into play", it triggers "when X come into play" abilities, but NOT "when you play X" abilities, since it's being put directly into play as part of the abilities resolution, so the ability itself goes onto the stack, but whatever is coming into play never does. For instance, if you activate the Soulstoke's ability, only AFTER it resolved you opponent gets to see WHAT is coming his way. He has to decide wether to, say, Trickbind it BEFORE you show him the Nova Chaser.