Not true. The cost of scouting new provinces increases with every province you scout, and decreases with every Advanced Scouts tech you research. Upgrading the Main Hall does not increase scouting costs, it merely allows you to store more Coins and thus to pay for the scouting of far-away prvinces (for example, you can't pay 5 million to scout a new far-away province if your max storage is just 4.4 millions)
I think I may have found the relationship between upgrading your Main Hall and the cost of scouting provinces that I was referring to earlier. Point of information: I am toward the last part of the Woodelves chapter and have been working there across a few events (several months).
I know I saw something somewhere a time back, so I intentionally stopped upgrading my Main Hall (and basically forgot about it and why, knowing I would eventually get to a point where I had to start upgrading again, and that would be fine). I have continued to scout, basically on a continuous basis. I know, "bad player". I have advanced beyond the limitation set by Inno to discover Orcs before going past a certain ring (I believe ring 5) on the world map for scouting. At some point, this will probably bite me again. But it works for me at this point.
So, my coin cost for scouting each new province has been going up each time as expected, but I was not monitoring that cost until recently, when I realized that the cost was getting relatively close to the capacity of my Main Hall. Untill I measured this, I was expecting the cost to continue growing past my storage limit and this would eventually force me to upgrade. Not wanting to get in a bind, I started to record the cost of all the provinces that I had already cleared to scout, but had not actually scouted. With the recent event, I had opened up a lot of said provinces and I can look at the costs each time I finish scouting the previous one. I noticed that, at this point for me, each province scouted increases the cost of all discovered / un-scouted provinces by 100K. That is, with the exception of the provinces that have already reached my max storage amount from my main hall. As I continue to scout new provinces, those stay at the max (not increasing as before), as the others increase until they in turn reach that max.
So, I believe what I read somewhere was that the MAX cost of scouting will increase each time you improve your Main Hall up to that storage capacity. That means that you can artificially keep that amount lower by restricting your Main Hall growth. It makes things a bit less convenient, because you need to max out the amount of stored coins each time you scout, but you are spending less total , in the long run.
Not that it really matters though, gold coin seems to be the least important resource in the game.
Just thought I would point this out so that others don't wrack their brains if they came across the same post somewhere.