If you build a GB without help you pay the full amount.*
If you use a Swap Thread you pay the full amount... but with potential bonuses (FP packages and blueprints).
Never said otherwise.
*Anyone can inspect your GB list, so this rarely happens Spontaneous contributors are common.
If you are content to make slow steady progress then the swap thread route may work for you. I like the flexibility (today I won 100 FP in GE, tomorrow I may have nothing to spare) and the chance to advertise my open GBs to guild members looking for blueprints. No pressure, no hassle.
As for the Arc approach, one could argue that it pays for itself eventually, but there is a pretty hefty upfront cost in real estate and goods. Then there is all the time, effort, and FP it takes to get that Arc up to Level 80 where the 1.9 calculation finally comes into its own. So not exactly free. More a question of priorities.