Hi,

I have to agree that some linear bearings are overkill on most homebuilt hobby machines. Accuracy and repeatability are a function of the entire system of components in your machine. Expensive linear bearings won't compensate for deficiencies elsewhere, so IMO money, design consideration and effort should be distributed evenly between all the components of a machine.
As far as weight goes, IMO the gantry should be as light as possible without sacrificing stiffness, but the table itself should be heavy to dampen vibration (assuming moving gantry style).
Back to the linear bearings, the Thomson bearings themselves aren't that expensive, it's the pillow blocks, case hardened shafts and shaft supports that are expensive, and with a little extra work you could use regular steel shafts and build the other parts. CAD files are available from most of the manufacturers, and you could end up with close to Thomson performance relatively cheap.

Steve