Your take on the ending (spoilers)...
I think it's supposed to signify that his last mark really had no family of her own (as is apparent when her boss asks her to take time off to spend with them) and that she really fell for Patrick too (mutual feeling), especially after reading up on his rough childhood and only petty crimes he had committed which put him behind bars in the first place. She sent the money back to indicate A) she wanted him to get a fresh start in life and B) that that she knew where he was and maybe she would come join him some day; that they may have a future together.
Any other thoughts?