Space battles are actually a bit cheaper and easier than you might expect. Usually, they boil down to ships just moving around against a background and they still look amazing if you do it right. Battlestar Galactica had massive space battles almost every episode without exploding their budget (they blew it on music instead). It's the same reason video game devs can afford to put so much more detail into the graphics in space games. Spaceships don't typically have a lot of (externally visible) complex moving parts that need to be delicately choreographed, and space typically doesn't have a million environmental objects around you that you need to render.
An alien planet, on the other hand, would take quite a bit because then you're having to go through the effort of blending the CGI to look good with the live action, and it can easily become a jarring mess.
I would totally go for something like Ben 10: Alien Force with a ton of blood and gore. Heck, Ben 10 is probably the closest thing we'll ever actually get to a good Animorphs adaptation.
