The affix ghañ comes after the root grah 'to seize' when the word pra is in composition and the sense denoted is the desire of acquisition.,
As पात्रप्रग्राहेण चरति भिक्षुः पिण्डार्थी 'the beggar walks for the sake of alms, carrying a vessel in hand'; स्रुवप्रग्राहेण चरति द्विजो दक्षिणार्थी 'carrying a sacrificial ladle, the Brahmana walks about for his fee'. The word लिप्सा means 'the wish to have something'.
Why do we say 'when meaning desire of gain'? Observe प्रग्रहो देवदत्तस्य ।,