There are many instances of characters characters "paying" for happiness in this novel.
The people of Struen give up their children and, in many cases, their wives in the name of normalcy. Any child born with an abnormality is sent to the Fringes. For many parents, like David's aunt, the knowledge that you are likely sending your child off to die is agonizing. If a woman in that society gave birth to 3 children with an abnormality they were also shunned by their husbands and sent off to the Fringes.
Individuals sacrificed love and friendship in an attempt to be "normal". Anne gives up the group, including her sister, and shuts down her telepathic powers in the name of love and the desire for a "normal" life. Anne also ends up betraying the group to her new husband in the hopes of finding happiness with him.
Uncle Axel pays for his love of his nephew with his own morality by killing Anne's husband to prevent him from revealing the secret that they are all telepaths.
David pays for his love of his friend Sophie by first hiding her deviance and then later staying away from home to prevent his father and the others of the town finding the girl. The cost is a painful whipping and the scorn of his father. It was all for nothing anyway because Sophie is caught and sent to the Fringes anyway.
The novel ends with the ultimate cost of happiness. Even though David and the others who have escaped are on their way to a new land where their abilities are valued, it cost the lives of Sophie, Gordon, David's father, the other invading Struen townsmen and all of the inhabitants of the Fringes camp. David may have won a lifetime of freedom and perhaps happiness with Rosalind and Petra but he has lost his parents, his other sister, the other telepaths and his Uncle Axel.
There is always a price to pay for happiness. In this novel it is a terrible cost.