This is my thoughts on a covenant economy boost. How does this sound?
The prophet gets to build medium turrets right away. This will help him deal with the early hog issue a lot better and medium turrets will not be a waste of supplies to have late game
Chieftain could get half prices UNIT upgrades. (Not for chieftain himself, only those in the factory, hall, summit, and for brutes. This would being him back into non-rush play, wouldn't affect the brute rush even slightly and would make him at least a decent choice in 3 vs. 3.
Last, the arbiter is fine as is economy wise. I think a 25% damage increase to elite honor guards both in and out of rage mode would help make the arbiter able to be used to counter an honor guard rush.
I didn't really think all that through too much but I think giving the covenant these bonuses would make them able to compete with UNSC a little more easily.
~Trueskill 40~
Add me- Gamertag: xNiitr0x <--its a zero