This cube has Invasion block and both Ravnica blocks. That's a lot of cards. It can use Eternal Masters, Vintage Masters, and both Modern Masters sets as core sets. I added packs from Ravnica block first ($10 worth), and then I added $15 in singles since the singles are cheaper than the packs for Ravnica block. I also added singles to Invasion block. Here are the packs.
First, a Ravnica pack, because I can pull something awesome (I didn't, and that's OK).
Then a Dissension pack.
And then six packs of Guildpact.
I am missing a small number of cards from Invasion and Ravnica. I already have the Return to Ravnica block complete online. Since I have the shocklands from Return to Ravnica, that's ten missing cards I don't need. I also have a few Ravnica block cards as reprints in the two Modern Masters sets.