If for example you and someone else both pay Dropbox for 50GB each - ie Dropbox is being paid for 100GB of unique storage - and then you share some folders they deduct their space from your quota. As an example if they share 20GB of folders with you then you'll only be able to have 30GB of your own unique storage and between the two of you there will only be 80GB of unique storage.
Do paid sharing with a few more people and you'll quickly be out of space.
But they do the same counting for paid accounts even when all parties involved are paying. No amount of discussion with support would get them to even acknowledge the issue. If you go into your account settings you'll see the bar graph and details of how shared files are being counted against you.
i just need another 500mb, and i just dont need to pay another $10/m for a bunch of disk space i won't use. everyone i know already has dropbox too, so referrals are out.
I hope the pricing of dropbox lowers soon too.