McMaster ballscrews

Greetings to you all;

Thumbing through the McMaster catalog I noticed some weird pricing on their
ballscrew assemblies. For instance -

1/2" screw at $2.21 per inch. The nut is $129.26.

5/8" screw at $1.02 per inch. The nut is $19.50.

3/4" screw is $1.68 per inch in .25" and .5" lead, but the .25" lead nut is
$34.57 whereas the .5" lead nut is $131.31.

What gives?

Also, has anyone used these screws? The catalog doesn't say what level of
preload, if any.

Doug Harrison