To test the ability, I chose these 3 maps; Crimson Valley campaign, Lost Jungle campaign, and the Underpass campaign.
I built 1 Assassins Guild first and upgraded it with level 1 Pickpocket, and maxed Dodge/Counterattack.
Immediately after that I played with the same layout again, this time without Pickpocket.
Then I compared the gold left at the end of the game. Of course the gold earned from calling early waves is deducted.
Crimson Valley campaign: difference 113
Lost Jungle campaign: difference 35(1st session), 44(2nd)
The Underpass campaign: difference 168
It made negligible differences and the investment is recovered around the end of the game. And
the more your defense is strong, the less they contact physically with enemies, resulting less stolen gold.
There is a fun fact in it but its merit is very little considering Assassins are necessary only on a handful of levels.
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This time I used Buccaneer's Den campaign map. and put 1 Assassins at the front of the ship.
Only then it made a notable difference; 512 gold.
However, generally no one would place barracks forward and make them face enemies first. And upgrading barracks to lvl 4 usually happens in the midst of the game.