In Matthew 16:26 (21st Century King James Version), Jesus asks,
For what is a man profited, if he shall gain the whole world, and lose his own soul?
Sir Thomas More's quip in A Man for All Seasons contrasts the grand folly of losing one's soul for the world with the petty absurdity of making the same loss for a small and relatively insignificant piece of the world. Richard Rich, he points out, has sold himself short by committing perjury and betraying his friend for such a trivial prize as the Attorney Generalship of a principality.
Beyond this, there are two further points of significance. Wales has been the butt of English jokes since time immemorial. In the Middle Ages, the English treated the Welsh as a foreign subject population who were barred from holding office in church or state. There were frequent uprisings, culminating in Owen Glendower's rebellion at the beginning of the fifteenth century. Anti-Welsh prejudice would therefore have been familiar to Sir Thomas More, with many further instances over the succeeding centuries being known to Robert Bolt.
Secondly, the Tudors were of Welsh descent. Henry VIII was only the second Tudor monarch, and the Tudors were still seen as upstarts by some of the older English families, particularly those closely connected with the Plantagenets. Snobbish dismissals of Wales as a barbarous place, therefore, could be taken as an indirect attack on the king and his family.
If, as the Bible tells us, it profits a man nothing to sell his soul to gain the world, then what does that say about a man who sells his soul to gain the position of Wales's most senior law officer? That is the question with which Sir Thomas More confronts the treacherous Richard Rich in A Man for All Seasons.
Sensing an opportunity to ingratiate himself with King Henry VIII, Rich has shamelessly betrayed Sir Thomas. For his troubles, he has been rewarded with the position of Attorney General for Wales. At that time, Wales was a relatively small and insignificant part of Henry's kingdom, and Sir Thomas is utterly incredulous that Rich would sell his soul to become the king's chief law officer in that particular neck of the woods.
The implication of Sir Thomas's exasperated question to Rich is that his betrayer has sold his soul for an absolute pittance. That Rich should've done so for such a relatively minor honor is an indication of just what a shameless careerist he is and what little self-respect he has. Unfortunately for Sir Thomas, and for England as a whole, Rich is not the only one to abase himself at the feet of the king and sell his soul for personal advancement.
The brilliant play A Man for All Seasons by Robert Bolt tells the story of the conflict between Sir Thomas More and King Henry VIII over Henry's proposed divorce of Catherine of Aragon so that he can marry Anne Boleyn. As a matter of principle, More refuses to condone the divorce and remarriage. The play presents More as a man of the utmost sincerity and integrity who will not compromise on matters of conscience.
In contrast, the character of Richard Rich is concerned only with the acquisition of wealth and position. He has no desire to be a teacher, as More suggests, but instead is willing to do anything to increase his stature among the powerful. More attempts to convince Rich to maintain his integrity, but Rich doesn't listen. He eventually betrays More to his own advantage.
The quotation in the question is taken from chapter 16, verse 26 of the book of Matthew in the New Testament of the Bible. In it, Jesus says, "For what is a man profited, if he shall gain the whole world, and lose his own soul?" A similar quote is found in Mark 8:36. Since Wales, which is a small part of the British Isles, came up in discussion, More is pointing out to Rich that since a person's soul is worth more than the whole world, certainly it isn't worth it to compromise principles for something as insignificant as Wales. It is another attempt on More's part to convince Rich to maintain his honesty and integrity, but it fails.
In the struggle between Sir Thomas More and Henry VIII, More quotes the Bible passage (Mark 8:36) “What does it profit a man if he gains the whole world but loses his soul?” as an admonition to Henry for his abandonment of the Roman Catholic Church in order to get a divorce (which would then allow Henry to marry and thereby get Wales under his power.) More’s quotation is to show how it is a mistake; he compares the primitive, barely worthy Wales (not worth losing his soul) to the “whole world” in an ironic chastisement to Henry (actually addressed to Richard Rich, a court lackey who is trying to get in Henry's favor by taking his side.)