they already paid and ahev blueprints in hand, more so they just need a builder. the house isn't anything too extravagant, but they were given the est. of $560k, which is a bit much.
Too much based on what? How much it should cost, or how much they want to spend?
Most builders, and i'm going to assume log home builders will do it the same, will give you an estimate off the top of their head based on square foot, # of stories, bathroom quantity, foundation type, etc., so if your parents have plans in hand they can probably get a verbal estimate over the phone.
I just went to a log home builders site really quick and found that they're quoting a traditional log home goes for about $100/sq.ft. + land. garages and basements are about 45$/sq.ft. Given the upper michigan area is "remote", that will possibly be a cost driver. Type of logs is a big one too. In the south they will build out of pine, and i've heard other companies won't do that.
IIRC, traditional stick built homes go for the same, of course depending on the area. West Michigan right now sucks if you're selling, but buying is great. Building never fluxuates, unless the builders are really hungry for work, which they are here.
I believe Ken Hill Log Homes is local to West Michigan. If he builds up there maybe he can give you an estimate, but if not, and assuming he's cool, he will refer you to someone good in the area.
Good luck...