If this is an early feudal economy (I realise most games don't have this setting, but it's worth thinking about), then there simply isn't that much money sloshing around. Nobles are generally paid in food (the peasants' harvests) and services. Most peasants hardly have any money at all (they provide their own food and clothing, if there isn't a bad harvest, and housing is granted by their Lord of the Manor). Even the King/Queen doesn't have that much money either, as his/her income comes from a % of the nobles' income (the aforementioned food and services), and from the royal estates, where again, the income comes from goods and services. There'll be a bit more money in the towns, but not that much, as even most townspeople grow their own food, until later centuries when towns increase in size and it becomes more of a cash economy.
You can use this as a reason not to give out much money (and the PCs won't have many problems due to lack of money, because money isn't the be-all of this society). However, if you do allow the PCs to bring back hoards of treasure from the ancient ruins, this could easily make them super-rich, and would possibly wreck the economy or even the entire society, depending on how they spend the money. That's not necessarily a bad thing; it actually makes for a much more interesting game than "spending it on a magic shield" or flipping goggles*.
There wouldn't be that much to buy in such a society, but hiring servants/soldiers/craftsmen/tradesmen would be something they could do. They may quickly become wealthier than most nobles, or even the ruler**, allowing themselves to set up as serious rivals. But they may attract negative attention very quickly - why are they hiring most of the area's skilled builders? Not only is it worrying that they are planning to build a fortress, but they are diverting labour that is needed for the King's castle/ or the Bishop's cathedral. Anger, chaos, trauma, angst and inconvenience ensues.
It all depends on how the campaign is set up, really.
*hilarious call-back to the other thread about magic items.
**in cash terms; the ruler has the fealty of all those armed nobles, knights, and by extension their men-at-arms. "Wealth" in such a society is a bit more nuanced than just cash value. Of course, PCs who an hire large number of mercenaries might find the less loyal nobles defecting to their "noble" cause, and we might have a major civil war in the offing.