I have a love-hate relationship with the BTG
I built it when I unlocked it in my first city. My goal at that point was to get all the AW's that came with cliff art. I didn't upgrade it past a couple levels (or the Crystal Lighthouse) and sold both of them off somewhere in the mid chapters when I wanted more military resource focused AW's. I move very slowly through the research tree when I am moving, lol. I'm parked atm in both cities. Also, when PP's came out, everyone got them, even early chapters that didn't know what they were. There was no crafting yet, so no spell fragments or a disenchant button, so I have a huge inventory of those. A couple years goes by; I've considered the BTG a couple times since the chapters get slower and slower, but had avoided rebuilding it. I got to Ch17 and decided I wanted to complete the chapter for the Dragon Ark AW at the end. Then I found out there were nowhere near enough shrimp in all the oceans of Elvenar, rebuilt the BTG and upgraded just past the 2nd rune stage, so lvl 11 or 12 now? I never built it in my 2nd city, but I'm parked at the beginning of Ch16 there. I'm sure I'll build it there before I decide to move in the tech tree again, lol!
I don't know what it's like moving through earlier guest race chapters anymore. If I had it to do over I'd have just kept it. I would not have prioritized upgrading it, but might work on a level here and there. Having it started would have helped; I'd have probably seen the benefit sooner and upgraded it as I got into higher chapters.
I would never build it for the wholesaler discount, though that's absolutely a good reason especially when the decaying goods come into play. I think that's why so many later chapter players began recognizing the value and building it. Getting extra guest race goods was probably more of a 'bonus' reward for many of them. I'm just odd:
I truly hate the wholesaler, lol! I won't buy from him outside of event quests. For anything. Ever. I use time boosters to burn off excess supplies in troop production and buy kp with excess coins. My next kp will cost 2104K coins, I'll buy 5-10kp/day at that ever increasing price, and I still call the wholesaler a rip-off, lol! Guess it doesn't have to be logical; have I mentioned I. just. don't. like. him?