Holon Legacy is good in certain situations, but it is not widely used in highly competitive play, unless in a completely rogue deck.
Holon Farmer is definitely a good catch. Almost all decks that utilize the Holon Engine (using Holon Transceiver, Holon Mentor, etc....) also use Holon Farmer in order to get used cards back for possible play.
As far as their worth...Legacy isn't worth a lot, maybe $2.00, and that might be pushing it. Farmer is worth anywhere from $3.00 to $5.00, because of their wide usage and demand.