I maxed out Vette, Jaesa, Talos, Khem, and Xalek. Rank 5 gifts that they actually liked were few and far in between, and were almost always extremely expensive. They only give big affection numbers if there's a significant difference between your current affection level and the rank of the gift.
5-6k credits for a gift that gives 48 affection is not a great deal, and you'll mostly find those on the GTN, especially when you factor in that there aren't that many worthwhile professions, so the comps that gives bonuses to bioanalysis/slicing/etc drive up the price of gifts that they favor.
At 8000 affection, rank 5 blue gifts should give 160 and rank 5 purple gifts give 384. With the legacy perk that adds 30%, then the blues give 208 and the purples give 499.
If you're only gaining 48 affection, you're giving the wrong gifts.
Vette likes Underworld Goods
Jaesa likes Weapons
Talos likes Cultural Artifacts
Khem likes Cultural Artifacts
Xalek likes Weapons and Military Gear
Here's a spreadsheet with all the data you need: