Thirty pieces and not a coin less
Thirty pieces of silver is a Biblical reference. In Exodus, you can read that thirty pieces of silver was the going rate for a slave (Exodus 21:32). Judas betrayed Jesus Christ for 30 pieces of silver. This is how much he was given to turn Jesus over to Pilate's soldiers in the Garden of Gethsemane. In the play Raisin in the Sun, Beneatha teases Lindner with this reference when he offers money to keep the Younger family out of the neighborhood. Thirty pieces of silver is also sometimes referred to as "blood money" - so it fits into this part of the play because Lidner's money is also "blood money."
Read about the play here on enotes.
In referring to thirty pieces of silver, Beneatha suggests that Lindner's offer to buy the family's new house from them is a bribe or "ransom" akin to the one that Judas accepted as payment for betraying Jesus Christ to the Romans.
At this point in the play (Act II: iii), Mama has purchased a house in a "white neighborhood."
"Lena Younger uses her insurance money to buy a house. She has bought it, however, in a segregated area, and though she is willing to face that battle when it comes, the ominous appearance of Lindner, who wants to buy out the Youngers to avoid their moving it to Clybourne Park, threatens future difficulties" (eNotes).
Residents of the Clybourne Park neighborhood have collectively decided to make an offer to the Younger family.
Lindner is sent to deliver the offer from his "people" and to deal with the Younger family, which he repeatedly addresses through the play's last acts as "you people," objectifying the family in this way and underscoring the categorical, impersonal, race-oriented point of view that he and his offer represent.
"Our association is prepared, through the collective effort of our people, to buy the house from you at a financial gain to your family."
Throughout this scene, Lindner defines his community as being hard-working and decent and relatively understanding. When he finally suggests that these qualities have led that community to make a "generous" offer that will collectively serve to maintain racial barriers, he does not realize either the irony of his statement or the fact that the neighborhood's decision expressly mimics the systematic racism that the family is struggling against.
His neighborhood group seeks to unify against change in racial dynamics in Clybourne Park. The racism inherent in this point is lost on Lindner, who feels that somehow his hard-working community should not logically or effectively include other hard-working families like the Youngers. The fact that this discriminatory action is intended to be undertaken via purely financial means underscores the troublesome economic reality of a society that would keep one demographic down or excluded by utilizing its major advantage - wealth (and access to wealth).
Lindner's proposed action functionally betrays both the neighborhood's avowed virtues (by repeating racist exclusion-ism) and the Younger family's moral sensibilities (as each family member strives to find a way to live with dignity and hope in a social world that too often works to deny them both dignity and hope).
Thus, when Beneatha responds to the proposed buy-out with the bitterly sarcastic line, "Thirty pieces and not a coin less!" she is recognizing the ironies of the situation that Lindner cannot or will not see.
A further irony of this scene comes in the immediate solidarity the family takes on in the face of Lindner's offensive offer. Where Walter and Beneatha had been opposed to the idea of moving into a white neighborhood and felt at odds with one another as well, the brother and sister unite in their family pride after Lindner comes and attempts to insult that pride.
We can note that this family pride is exactly what is at stake through much of the play and is precisely the object that embattles Lena Younger and her children. She sees reason to be proud, despite hardship and limitations. They see a need for worldly recognition, a shift in material circumstances, or even a new foundation of identity as necessary for a new basis for family pride.
When Lindner suggests that they betray their mother and her efforts to overturn (or, more simply, to ignore) racial boundaries, the family pride that Lena had argued for suddenly and reflexively comes to the fore for Beneatha and Walter.
While this moment of family pride proves to be somewhat short-lived, it makes a return in the play's climax - a fact that renders the scene of Lindner's offer as a salient thematic moment in the play, demonstrating the nature of one of the thematic conflicts in the narrative.