Crew cost

Crew cost at the starbase

All crew cost related information from the game source:

Crew costs 3 RUs at the start of the game.
When you've recruited 1,000 crew or more (minus the amount you've returned), the crew cost is raised by 2 RUs. If the total later comes below 1000 (by returning crew), the crew cost goes back down by 2.
There is no actual limit to the amount of crew you can buy. Commander Hayes mentions that there are "almost 2000 highly-motivated, skilled professionals aboard this starbase" available to you, but apparently there are also an infinite number of unmotivated, unskilled amateurs at your service.
When you've made an alliance with the Shofixti, the crew cost goes down by 2 RUs. Also, the number of crew recruited is set to 0 and will never be raised (or lowered) again.
When you've sold no more than 100 crew to the Druuge, there's no change in the crew cost. However, Commander Hayes talks about "rumors".
When you've sold between 100 and 250 crew in total to the Druuge, the crew cost goes up by 7. Commander Hayes talks about "evidence".
When you've sold more than 250 crew in total to the Druuge, the crew cost goes up by 15. This is in addition to the previous cost increase associated with selling over 100 crew. Commander Hayes calls you a "heinous slave-trader".

If you do your shopping with the Druuge using crew as semi-hard currency, here is the price list:

Mauler ships: 100 crew a piece
The ship is worth 1700 RUs and comes with 14 crew.
Artifacts (Wimbli's Trident, Glowing Rod or Rosy Sphere): 100 crew a piece
Fuel: 10 crew per 10 units of fuel
The normal fuel cost is 20 RUs per unit, or 1 credit at a Melnorme trader.