My original motivation for watching the movie was--?-- the cool fight scenes and the exotic alien monsters.

This is tricky territory. I think you used "was" to match the subject "my motivation."
However because there are multiple scenes and monsters, the singular verb "was" somehow doesn't seem right. One solution might be to use "were" and you could run that through a grammar checking site to see if it would be ok.
Sometimes, when I have this kind of dilemma (and English is my native language!), I use a different sentence structure to avoid the problem.

What do you think about the price?

The ticket is €16 (~$19) at the cheap times and €21 (~$24) at the expensive times here

It's a decent commercial film, not very hard-core, but that's fine, considering its PG-13 rating, which was meant to appeal to a mass market.

Too many pauses in a sentence makes it a feel a little disjointed and little hard to read, so I took out some of the commas