Of the two million, half are inactive. Of the rest, most are not close to 2gb or use it lightly, so it´s not that expensive.

Things like LAN-sync will also help driving down costs.

Overall, I think that even if they are still burning money, they could stop doing that and go positive quickly.

2gb of storage costs $.24 a month on S3, and you figure they only average about half full. Call it an even $.50 with data transfer costs, and you've got just one paying customer (at $9.99/month) subsidizing ~15 free users. A 1-in-15 conversion rate seems like a modest goal. They may not be losing any money at all.

I use dropbox and am happy with it. I save files to it every day. I share files using it about once a fortnight and I grab files via the web app about once in 2-3 months.

I'm at 23.1% of 2.5 GB.

I don't think I will upgrade soon unless I start to use two machines regularly.

I would guess less per average free user (maybe as low as $2 per year). I would guess that 1/15 conversion is also a high estimate. But I might convert in a few years time. I imagine that it'll take another year or two before they really know what conversion rates look like.