High Level Great Buildings and The Arc

You may have noticed how The Arc has taken over a large portion of the top of the Great Building rankings and when you look at it at first glance there's really no doubt that it is indeed worthy of the title Great, but the obvious questions comes up: how far should I take my Arc before I focus on my other objectives? Below i have a chart of the non-refunded FP cost of completing one level of Arc at various tiers when the donors all have Arcs of their own:

Donor	Levelling Arc
	10	19	29	39	49	58	68	80	90
0	790	802	865	1010	1265	1618	2189	3200	4404
10	726	666	640	692	850	1111	1577	2456	3547
19	708	626	575	600	729	964	1399	2240	3298
29	688	582	503	498	595	801	1202	2000	3022
39	667	538	430	395	461	637	1004	1760	2745
49	647	494	358	293	327	474	807	1520	2469
58	628	454	292	200	206	326	629	1304	2220
68	618	432	256	149	139	245	530	1184	2081
80	606	406	213	88	59	147	412	1040	1916
90	603	402	205	77	46	130	392	1016	1888

Of particular note here is first off the diagonal bold line of entries.  This represents the true levelling cost (with refunded forge points subtracted) of an arc via fair deals with similarly levelled arcs.  Notice by level 58 how it's less than a quarter of the no arc cost.  At level 58 two things happen that cause it to start tailing off.  First is the arc drops to only 0.5% contribution boost per level.  Second is that while the FP cost of next level is a slow exponential growth of 2.5% per level, the refunded FP increases linearly from level to level and past level 50 the exponential growth becomes faster than the FP growth.

Also of note is the line across for high level donors - this curve holding for nearly all buildings they donate to.  With such a small 'true cost' compared to the total cost, and since the majority of refunded FP are held by the top two spots, it doesn't take much wasted FP for a high level arc to hijack a level 29-58 building.  It takes even less for two coordinated FP donors with high level arcs to do so - they may even do so without a single point having been donated yet!  They may do this for profit or they may do it to seize your building from going further as the remaining 3 spots won't offer much incentive for donors to contribute.  As such, it can be important to have your swap partner lock first place immediately after levelling the building!  Save your packs :)