Breach the Multiverse costs seven, has each player mill ten, and you can choose a dork/planeswalker from each graveyard and send it to your side
Not bad for such a little guy, is it
Regirock can be obtained from Space-Time Smackdown: Dialga and Space-Time Smackdown: Palkia packs in the Space-Time Smackdown set
Mayaels Aria is also worth bringing up since its the most accessible price-wise in a decade after its Surviving to Upkeep One thing you'll notice that all these cards have in common is they trigger at the beginning of your upkeep, which poses the inherent problem of projecting your win condition to the field potentially multiple turns before you can pull it off
Once you have duals you can upgrade your ramp to things like Farseek and Nature's Lore
In fact, I would tend to prefer a 3-mana Walker that does this with a +1 over a 5-mana Walker that just draws a card and mills one (Jace, Memory Adept)