SWTOR Buying Companion Gifts

In SWTOR (Star Wars The Old Republic), at Companion Influence level 1-39, the most cost efficient gifts are Rank 1 Green (sold by vendors for 200 credits). At Influence level 40+ the effect from gifts is lowest, and most cost-efficient when you use Rank 5 Blue gifts.
Here are charts showing the relative values of gifts, and therefore how much you should pay if you are looking to buy. Despite Rank 5 Pink gifts being slightly overpriced you should still consider them simply because of the time savings involved.

Influence Gain at Influence 40+ (without Legacy Perks)
We based our numbers on the excellent Dulfy article on version 5.0 Companion Influence in SWTOR.
Legacy Perks, which can increase Influence Gains by up to 30%, will influence these numbers somewhat but not by much.

Rank	Green	Blue	Pink	Yellow
1	0	0	0	0
2	13	26	51	128
3	26	51	102	256
4	51	102	205	512
5	77	154	307	768
6	102	205	410	1024

Influence Gain Relative to Rank 5 Blue Gifts

Rank	Green	Blue	Pink	Yellow
1	0.00	0.00	0.00	0.00
2	0.08	0.17	0.33	0.83
3	0.17	0.33	0.66	1.66
4	0.33	0.66	1.33	3.32
5	0.50	1.00	1.99	4.99
6	0.66	1.33	2.66	6.65

Relative Value of Gifts

Rank	Green	Blue	Pink	Yellow
1	0	0	0	0
2	422	844	1,656	4,156
3	844	1,656	3,312	8,312
4	1,656	3,312	6,656	16,623
5	2,500	5,000	9,968	24,935
6	3,312	6,656	13,312	33,247

Vendors exist for the following:

• Rank 1 Green - 200 credits
• Rank 2 Green - 600 credits
• Rank 5 Blue - 5,000 credits
• Rank 5 Pink - 10,000 credits
• Rank 6 Yellow - 250,000 credits

Other Influence Ranks

At different Influence Ranks, different Gifts are more cost-efficient (though definitely not time-efficient in terms of actually giving the gifts):

Short of someone dumping gifts cheaply on the Galactic Trade Network, the cheapest way to get gifts is generally Crew Skill Missions which can get you a Rank 5 Blue gift for under 2000 credits, and a Rank 5 Pink gift if the follower has a critical success.