Originally posted by montecarlo non-IS Canon 100/2.8 USM 509 USD (with IS 944 USD... but that's an L :-) ).. yes.. mea culpa.. I didn't checked for the exact prices
$509 for an imported, grey-market lens with no manufacturer warranty. $519 for the actual, warranted item. (and the Pentax can be had for $10 less at Adorama, too, cancelling out some more of the difference.)
And that Canon not only lacks IS, it lacks WR, has a consumer plastic housing and a mixture of plastic / metal internals, versus a metal one for the Pentax, and yet it is still MUCH bigger and heavier. (600g / 3.1"D x 4.7" L for Canon, 340g / 2.6"D x 3.2"L for Pentax).
It does have one advantage that I'll give you -- the quieter USM motor. Other than that though, the Pentax (on paper) outperforms it handily for somewhere between 15-20% more cost.