Volume 1 = season 1
Volumes 2-6 = season 2-3
Season 4-6 = that
Slices 1-4 = season 7
There aren't any repeats in the volumes/seasons. Volumes 2-6 have at least 12 episodes each, they're worth the price-tag. It's not at all like the Clone Wars where they put 4 episodes on each.
You'll pretty much need to buy all of the volumes and seasons to complete the series thus far.
Deepdiscount.com has had many B1G1Free kids shows sales with some of the volumes (never 5, though, I had to buy mine from Amazon) included and almost always seasons 4-6 (4 = $19.99, 5&6 $14.99/$7.50 each after B1G1Free). I'm hoping the next sale will include the slices so they'll be $5 each.
There are 10 (9?) seasons in all, but the last 2 or so are mostly rubbish. I wouldn't expect a full series set anytime soon, and if it ever happens, it'll be very, very expensive.
I'm currently watching volume 6 with my 2-year-old son, he loves the turtles. The dialogue is really starting to pick up in terms of quality and gut-busting laughs.
Casey Jones: "Who's the nutcase now?"
Raphael: "You are, but thanks for saving our shells!"