Olympus 4/3rds to Pentax K-5 primer/ system intro

If in fact they are the same lens, why is pentax version is much more expensive? just because of weatherproofing and sdm? then why don't they also make a cheaper version without sdm and weatherproofing, not everybody need sdm and weatherproofing. If the lens price continue to be this high then everybody will just buy sigma lens, sigma provides better lens selection for pentax user better than pentax itself, even tough their primes are not as small.