The airlines are collectively losing $200M a day. They're obviously not incurring flight costs (fuel etc.), but many aircraft are bought using lease financing which has to be paid and you still have to pay employees.
The particulate matter is actually extremely fine. It's mostly rock and glass (ie. not the nice fluffy ash you find at the bottom of your BBQ) which, when ingested into a turbine, melts and coats the inside. This makes the average jet engine very very angry and very very broken.
This is going to sound really girly, (and I imagine Daisy will know what I'm talking about) but there is this very fine powder made from coloured glass that you can use when making cards, and you stick it on using a stamp and then either put it under a kitchen grill, or use a glorified hair dryer to melt, so that you are left with a smooth raised picture. That is what happens when the glass gets into the hot jet engines I'd imagine. Also if the actually ash get wet then that will add a whole new dimension to it because you will end up with ashy rain (it probably has a technical term) that won't necessarily cause problems to airlines, but will reek havoc on the ground. That is what caused the town nearest to Pompeii to be covered in ash in one of the Vesuvius eruptions. And in Montserrat in 1994 it caused major issues.
That is another point, if Montserrat is anything to go on, and it like Iceland is a totally volcanic island, we could be in trouble because the eruptions there started in 1994 and continued into 1996... And Montserrat is much smaller that Iceland... I think it could be different though because the lava type in Iceland is quite different since it comes directly from the Earths mantle.