You pay for race cost independent of how whether the "character" is an individual or henchman group (which you actually decide later, when choosing a stat array) and the cost has nothing to do with how many henchies there are - it's the same with 5 guys as it is with 20. Each extra 5 warm bodies in a henchgroup costs 3 points, but that cost doesn't vary regardless of species. It's pretty much the same mechanic as buying equipment for them - you only have to pay for one blaster no matter how many henchies there are in the group - same applies to background traits, skills, traits, etc. The whole group is like one model during character creation.
The advantages of having an expensive race (or gear, trait, whatever) for your big 20-man mob is offset by their severe limitations against heroes (max 2 vitality loss per hit is pretty horrid), generally weaker basic stats (since they can't customize and their resolve stinks), and increased vulnerability to radius weapons.
Now, if you're wondering about mixed-race henchgroups, that weirdly never came up during playtest AFAIK. My advice would be to to just give them Extra Fate for 1 point. That makes them all effectively funny-looking humans, which is very appropriate for space opera - in some cases, you can probably even see the zippers on the costumes.