I can't get into the Steam store page right now, but yes the summer sale at this time last year was more like 80% off. Could still be a great deal for many, though.
In addition to having to purchase the base game over again, one must have the Steam Client program installed and it must be on Windows (not Mac as Steam doesn't carry TS3 for Mac). But that will be enough to enable the purchases of the EP/SPs and, instead of installing any of this, retrieve the Origin codes they then throw in free of charge. If one registers the extra otherwise unnecessary base game, one still gets 1,000 store points for it as that offer is still being honored in case that helps.
I repurchased everything and converted my entire TS3 install over to Steam last summer instead of using Origin codes so as to be able to get back on Patch 1.67 and no more Origin tie-ins. The base game and all EP/SPs (except Katy Perry which they don't carry and I didn't have anyway) at last year's prices cost less than $100 USD.