In addition to the others answers, I would add that Candy's willingness to "buy into" the dream makes it all the more sad and desperate. Candy hopes that the money will buy him the one thing he never has truly possessed: acceptance. George and Lennie already have this priceless commodity in each other. Man, Steinbeck continually notes throughout his work, is a group animal. To be excluded from the group (the phalanx) is certain death.
Because Candy has money saved up (he received money when he lost his hand), it makes the dream of the farm seem possible to George. Between George and Lennie, they never have more than $50 at any given time, so George never really believed that they would have enough money to buy land, but when Candy says that he will give them his money, George begins to believe they could actually do it.
Candy wants to be a part of their dream. He wants to be a partner, and will give them 350.00 of the 600.00 that they need to buy it. They all agree.