Well, to be sure, there's a balance to strike on the spectrum between "print your own money" and "beg for spare change." I'm not in favor of keeping Edge groups just scraping by, but I also don't think it's a great idea to let them get large amounts of money from downtime activities. To say nothing of how if you're giving the party a month between sessions and letting the mechanic immediately sell any droids they make, you're giving them many times the income a PC gets from say, Sound Investments, for a fraction of the experience cost (a few ranks of Mechanics vs. several talents). I don't want to keep groups poor, but I don't want to give them an incentive to not adventure by making it too profitable to stay home, and I don't want to make working as a droid tech too profitable relative to other downtime activities.
As for why I call 600 credits per monotask droid "non-trivial," that's less about the exact amount of payout the group gets for an adventure and more about competing priorities. My group got about 30,000 credits for our last mission, but we want to invest a significant amount of that in improving our ship (first thing we did with the money is buy an advanced targeting computer, since none of us have Gunnery), and then the remainder is split four ways, and then I have to decide how much money I can set aside for investing in improving my crafting versus buying personal equipment I can't just craft (welding rod, multi-goo gun, stimpacks and emergency repair patches, cybernetics, etc.). Since I would need to spend at least 2,400 credits to get a benefit out of helper droids, that's a significant chunk of the disposable income I've earned so far.
Just a sidenote: A failed investment imho. The gunnery droid brain properly modded cost zero hardpoints, less than 10k usually and has a gunnery skill of 4 which works with skilled assistance. The literally only downside is that the droid brain is a little trigger happy (classic quirk, which are represented in the droid crafting rules), so better not activate your weapons if you do not want to use them. It not like additional upgrades are not bad, but you still want imo that droid brain mainly, because skill 4 which can be used with skilled assistance is quite awesome for people without a gunnery skill.
Oh and imho the biggest limitation for crafting (outside of paying for your own workshop) is time. The credits might be trivial, even for your group with the 30,000 credits payout, it would be trivial to build combat droids or even just droid brains, but building all of it takes quite some time investments. And time can be a super, super spare resource for an adventure group which is constantly in the middle of the action thanks to all those duty or obligation roles or the empire and inquisition on their tails.
Oh and another side note because I have ,mentioned droid quirks: One suggestion/recommendations from Sam for crafting is to allow not to cancel out all threats to gain drawbacks on a crafting items while gaining additional advantages. Makes the whole crafting results more interesting, especially for the droids imo as the quirks are so highly entertaining.