I would definitely recommend asking the person making the claim for the calculations underlying it. I suppose, if you had an all-open exit policy, and were in a location where traffic was fully-metered and very expensive, you might be able to rack up US$1,000/day in costs, but that is very far from the common case. Even if you were running an exit in such a location, you can limit usage using accounting, which this person appears to be unaware of.
The nodes I run are on unmetered (but contended) Gbit connections, and they cost less than US$100/month for machine and bandwidth.
Since you've got experience with the other claim that this person made -- that a middle node costs over $100/day to run -- and you know it to be false, I think you've got all the data you need to conclude that whoever made the statement about exit nodes is speaking from their posterior, and can be ignored.