How much money does Candy have? Where did he get the money? Why is Candy so keen to join George and Lennie on their dream farm?

From the text:

Candy said, "I ain't much good with on'y one hand. I lost my hand right here on this ranch. That's why they give me a job swampin'.An' they give me two hunderd an' fifty dollars 'cause I los' my hand. An' I got fifty more saved up right in the bank, right now. Tha's three hunderd, and I got fifty more comin' the end a the month. Tell you what-"

Becoming a part of the dream means that Candy will have somewhere to retire... he will have companionship, and he will feel useful.