Came upon this one corpse, some axeman had taken his whole arm off at the shoulder. He was covered with flies, all crusty with dried blood, might be why no one else had touched him, but under them he wore this studded jerkin, looked to be good leather. I figured it might fit me well enough, so I chased away the flies and cut it off him. The damn thing was heavier than it had any right to be, though. Under the lining, he'd sewn a fortune in coin. Gold, Your Worship, sweet yellow gold. Enough for any man to live like a lord for the rest o' his days. But what good did it do him? There he was with all his coin, lying in the blood and mud with his fucking arm cut off. And that's the lesson, see? Silver's sweet and gold's our mother, but once you're dead they're worth less than that last shit you take as you lie dying.

The lesson Brown Ben got from that experience is to put his survival first above the bottom line, and he chose what he believed to be the winning side.

However, there is another lesson in that story that Plumm missed. All the gold sewn into that jerkin made it "heavier than it had any right to be" and likely slowed the sellsword down on the battlefield. The sellsword's love of gold kept him from enjoying it. The lesson is that greed can lead to a sellsword's downfall.

-ASOS, Tyrion IV

Some years later, when Lord Farman of Faircastle grew truculent, Lord Tywin sent an envoy bearing a lute instead of a letter. But once he'd heard "The Rains of Castamere" echoing through his hall, Lord Farman gave no further trouble. And if the song were not enough, the shattered castles of the Reynes and Tarbecks still stood as mute testimony to the fate that awaited those who chose to scorn the power of Casterly Rock.

-ASOS, Tyrion III

Tywin’s policy of compelling people’s loyalty was the spirit of a “a Lannister pays his debts,” which as well as pointing to the Lannisters’ obvious wealth is also a threat. Securing loyalty usually came down through two things: buying them off or intimidating them into submission. 

When Tyrion objected to Slynt pointing to him being a sellout, Tywin notes by being the wealthiest house in Westeros, no one can outbid them, so that actually works in their favor. However, the soundness of such a strategy is questioned directly in A Dance with Dragons. 

"So they betrayed me, is that what you are saying? Why? Did I mistreat the Second Sons? Did I cheat you on your pay?" "Never that," said Brown Ben, "but it's not all about the coin, Your High-and-Mightiness. I learned that a long time back, at my first battle. Morning after the fight, I was rooting through the dead, looking for the odd bit o' plunder, as it were. Came upon this one corpse, some axeman had taken his whole arm off at the shoulder. He was covered with flies, all crusty with dried blood, might be why no one else had touched him, but under them he wore this studded jerkin, looked to be good leather. I figured it might fit me well enough, so I chased away the flies and cut it off him. The damn thing was heavier than it had any right to be, though. Under the lining, he'd sewn a fortune in coin. Gold, Your Worship, sweet yellow gold. Enough for any man to live like a lord for the rest o' his days. But what good did it do him? There he was with all his coin, lying in the blood and mud with his fucking arm cut off. And that's the lesson, see? Silver's sweet and gold's our mother, but once you're dead they're worth less than that last shit you take as you lie dying. I told you once, there are old sellswords and there are bold sellswords, but there are no old bold sellswords. My boys didn't care to die, that's all, and when I told them that you couldn't unleash them dragons against the Yunkishmen, well …" You saw me as defeated, Dany thought, and who am I to say that you were wrong?

-ADWD, Daenerys VIII

As Brown Ben explained poignantly, sellswords often have their eyes on the bottom line, and usually will sell their services to the highest bidder, but when backed against a wall, they will always put their own survival first. Tywin wasn’t the only to use the golden rule of “he who has the gold rules.”  The Yunkish lords did the same in forming their coalition with many sellsword companies, but Symon Stripeback himself noted the limits of the extravagant wealth of Yunkai in maintaining the sellswords’ loyalty: “As a slave in Yunkai I helped my master bargain with the free companies and saw to the payment of their wages. I know sellswords, and I know that the Yunkai'i cannot pay them near enough to face dragonflame.”

Even if no one can outbid the Lannisters, people of bought loyalty will seldom stick around to to the bitter end for their employer as you can never pay a mercenary enough to have them practically sacrifice themselves for you. The Brave Companions Tywin himself hired switched sides when they saw that Tywin was losing up until the Battle of the Blackwater. 

The Freys may not care, but the northmen … they fear the Dreadfort, but they love the Starks."

-ADWD, The Turncloak

The Northmen fear the Boltons, but love the Starks to the point that Ned’s bannermen among the mountain clans are willing to fight the Boltons. Self-professed coward Sam Tarly faced a wighted Small Paul to protect Gilly. People are willing to face great odds on behalf of the people they love and the causes they fervently support in spite of their fears.