That dickhead salesman either has absolutely no clue what he's talking about, or he's lying through his teeth. Either way, he's completely full of shit. I'm running a coco SOG with 26 plants right now, with an ebb and flow system, and those 26 plants use anywhere from 2 gallons to maybe 3 gallons per day - all of them together. According to his fantasy math, I should be using 13 gallons of water a day. Jesus, even if you stuck your plants in pails of water instead of pots of coco, you still wouldn't use that much water.
I use Canna nutes myself, and I'm really happy with them. You're right, they have a high price tag, but one of the things I like about 'em is that I only use 1/4 to 1/3 recommended strength. If I followed Canna's feed chart, I'd have to cut a hole in the roof of my house because the damned stuff is so potent. Roughly 300 PPM of Canna is all I need, on top of the natural minerals in my tap water and whatever magnesium supplement I may be using.
You also don't need the full nute line. I just use Canna A and Canna B. I have some PK 13/14, but seldom use it because it has a tendency to bind with PH down and the calcium in my tap water to precipitate into calcium phosphate in my reservoir during flower. I use M.O.A.B. for a bloom booster. You can skip the Cannazyme, too - for a zyme, I just use Hygrozyme, which isn't cheap either but can be gotten a lot more cheaply than Cannazyme.
I don't know what kind of quantities you were buying, but even if you were buying it by the gallon that dude screwed you because you just do not need all that shit - and if you were buying it by the gallon, it would sure as hell last a lot longer than 2 weeks. I'm just finishing Week 8 of that 26-plant grow, and have only used a half gallon each of A and B so far.
If I were you, I'd buy your nutes online from now on. Normally I'm all for supporting a good local grow shop, but that shop plain, flatout cheated you. I'd never give 'em another dime.
Oh, and here's an example of what I got using just Canna A and B at about 1/4 to 1/3 strength. This was in Week 7, I believe, and those two colas you see tied down and angling lower left to upper right are 15 inches and 18 inches long, respectively.