1. Be the first to plant the flag on the new continent.
2. Money. They want to gobble up as much resources as possible. Hurry, discover new things--spices? Coffee? DRUGS? Salt? GIMMIE.
Okay, so, I think #2 here will give you a headache. "I want the main motivation be about money... but I don't wanna deal with money," isn't the best approach to compelling content.
The settlement backer's POV: Politely exploitative. The locals are useful, but once that's over, buy the land out from under them and shoo them off; the native races aren't close to human so intermarriage isn't possible, their numbers and tech level isn't enough of a severe threat (and they're not warlike), conversion isn't a concern, so just bide time.
You are playing D&D here? Their
technology is not the issue. Stone knives and bearskins are fine. In D&D, most of the damage comes out of the class, not the weapons. A couple druids or clerics of the native's gods with some barbarians to back them up could seriously mess up a town, if they were so motivated. And they have a continent full of them, while the settlement only has whatever they brought with them....
Indeed there was a previous civilization that was more advanced. It was wiped out because of bad things they did, and containing the fallout required heavy magical lifting from both nature spirits and extra-planar assistance. This area of the continent (The starter zone, basically) was walled off from the interior, and the locals considered everything associated with the prior civilization to be incredibly taboo--to the point some conservative tribes think building with stone is courting disaster; everything else made of stone got smote so why risk it? To them the stuff in the ruins is probably cursed or will unleash hell, so those weird foreigners are welcoming destruction on their own heads.
Hm. Sounds like you've created your own problem there, and I'm not sure about the idea that the natives will sit by while this goes on, but those are aside the point. Let us say the natives don't give a hoot about the loot. So...
Rather than contort yourself trying to figure out how to work normal economic function in a place that lacks it... just don't. Accept the thing you built, and its natural results. They will not be able to easily convert one form of wealth to another. So be it.
Use the idea of the "company store": The PCs bring gold, art objects, jewelry, etc from ruins or locations of exploitable resources to the settlement, they rack up
credit with the settlement. They can trade that in for the time of settlement craftsmen to make things for them. This will take time. Such is life. You can handle this all with normal GP values.
Also, recognize that in the end there's
no real difference between "give them random magic, and allow them to trade that in for what they want" and "just give them what they want in the first place". Makng them jump through a hoop to get stuff they like is not really a value-add, so to speak.
You'v epositioned it so that the PCs are unlikely to have anything the natives want, and have discarded teh idea of the PCs doign favors with the natives. Maybe just remove the natives as irrelevant? It is a continent full of monsters and beasts, and the ruins of a civilization that died....
This also gets you out of the bad look of re-enacting imperialistic oppression of natives... They just aren't there to oppress!