Jack - I was going to reply earlier, but I didn't really have any information so I held off - sorry. I think that Sigma multicoat filter is probably about the best price you'll find on a MC filter.
I had a protection filter on my bigma, but it wasn't MC, so I usually took it off when shooting. I see non-coated filters on ebay for as little as $10, but if you want to leave it on and shoot through it (I do this with most of my lenses), that Sigma filter's probaly a decent deal (though I haven't really shopped around). My main reason for attaching the filter was to protect the lens while traveling with the hood reversed in my bag - I wanted to protect the element in the event the cap came off... Another interesting option for this sort of protection would be a
Hoodie cap from lenscoat - pretty affordable and looks useful, but I've never used one.
Good luck.