Hi Søren
I have a mix of single and multi coated filters from Heliopan. And I have noticed a small difference if I point the lens and filter into the sun or another brigth light, where the single coated filter showed a little more reflections. Otherwise I haven't noticed any difference.
But despite the small difference, the new filters I am buying, are the multicoated type.
Try to check-out the price of the filters here
FotoMayr. The price of the multicoated filters at FotoMayr are "only" 1.5 to 2 times the price of the single coated.
Tom