...I actually got an amazon gift card and spent it on the boxed set...
If you want to see the extra one with Hohenheim, just buy the last one by itself (volume 27), at this point in the story it's exactly the same as brotherhood so you won't be confused about anything.
Okay so after reading some comments here and doing some math it's worth putting some things into perspective. Based off of the page counts from this post here are the Pages per Episode (PpE) for each part and total.
PHANTOM BLOOD 857/10 = 85.700 PpE
BATTLE TENDENCY 1257/16 = 78.562 PpE
STARDUST CRUSADERS 3045/48 = 63.437 PpE
DIAMOND IS UNBREAKABLE 3476/39 = 89.128 PpE
VENTO AUREO 3121/39 = 80.025 PpE
Total 11756/152 = 77.342 PpE
One Piece meanwhile is 18500/981 = 18.858 PpE
For the sake of comparison:
Full Metal Alchemist 5048/64 = 78.875 PpE (Page# is from the Complete Box Set minus the 5th Light novel that's included in it which is an additional 200 pages, it may still be inflated but I couldn't find another source. Episode count is from Brotherhood)
So clearly One Piece has a LOT of filler in some manner or another, it sounds like they just have it in every episode rather than having whole seasons that are skippable. Based on these numbers it looks like there is only actually about 5 minutes of actual content in each episode (20 minutes of content in each episode and 18.858 * 4 = 75.432)
No worries! I used to collect manga in Middle School and High School. I sold my collection barring a couple series. I am coming back into it after college...so I am not exactly the authority on deals, but I'm trying.
I am not too familiar with Rightstuf and stacking, someone else would be able to answer you a bit better. But if it is worth it to you to bypass the wait and get the manga, go for it! Usually Rightstuf is my last resort because I feel like I can find volumes much cheaper elsewhere with a bit of research.
Another deal you might want to look at is the Barnes and Nobles 3 manga for 2 deal. They come around every couple months. The last one was in December to January but it might come back around in the summer (June/Probably Middle July/Maybe August?). So...$9.99 x 2 + tax. = 19.98 / 3 = $6.66 per manga + tax.
I am not sure with Tokyo Ghoul since it's still being published by Viz and it's a newer series. It's gonna be a bit harder to find deals on them. I would see if you can grab the Publisher deal at Rightstuf or grab the Buy 3, Get 1 Free at Barnes and Nobles during their manga mania.
Similar case with HunterxHunter. But according to forums, last year, HunterXHunter was on the Holiday Sale for 2016 where they bundled like 7-8 volumes for like $40.00. I am not sure if Rightstuf will repeat Holiday sales in the future though.
But! I know for a fact that there is a Fullmetal Alchemist box set. -> (https://www.amazon.com/Fullmetal-Alchemist-Box-Hiromu-Arakawa/dp/1421541955) This box set has the whole series, mini-poster, plus a random novel. This box set I have seen for lowest around $110-$120 online. (So around $4-$4.50 a volume.) Barnes and Nobles has it new for $140 ($5.00 per volume). (Link: http://www.barnesandnoble.com/w/fullmetal-alchemist-box-set-hiromu-arakawa/1100163269?ean=9781421541952)
If you grab a coupon for 20% off, you can easily grab it for ~$115 + tax ($4.26 per volume). Barnes and Nobles constantly gives out coupons and I try to update the subreddit regularly with those coupons (or you can grab the subscription email, it's up to you). If you want to wait or you aren't in a rush to get it, for Black Friday the last couple years, Barnes and Nobles gives out 30% off coupons. So the box set will be $140 down to $98 + tax ($3.62 per volume). That's not a definite because Black Friday can be unpredictable but if you want to gamble and see if you can get that deal, it's up to you =)
Full Metal Alchemist also has Omnibuses out. They are 3-in-1s. There's 9 of them all together and they go for 14.99 a piece MSRP. I have found them in the past on Ebay/Amazon for a cheaper price (Around $10-$12 depending on the volume).
The box set seems to be on sale on Amazon right now for 159 bucks, that's less than 6 bucks per volume.