Trying to hunt for this one now... if all else fails and you need an answer in a hurry, go for around 100 credits a week plus 25 per experience level (which is about half that of a diplomat from the EA fact book at the same level, if I remember correctly)
in the pocket guide it states that a Profession skill check allows you to earn 100cr times skill roll result for one weeks work, i.e.: is my Earth Force Lieutenant is third level and has the skill Profession (Military) +4 and rolls a 9 he would earn 1,300 credits that week, the next week he rolls a 17 and earns 2,100 credits. the skill also says that untrained laborers can earn 25 credits per day
I have a house rule that goes something like this:
Profession (Military): the use of this skill for members of any standing Military force or permenant position requires the player to 'take ten' for their wages each week. their skill bonus represents increases in their payscale due to time in service and learning to manage their money better.