I would think that the costs of maintaining a listing for an ETF would be pretty minimal. You're looking at under $50,000 for your annual listing fee (I think it's $27,500 maintenance at NYSE), plus the annual audits, disclosure statements, etc. For a generally inactive ETF, those costs ought to be trivial (er, well, at least in comparison with mutual funds).
As long as you have assets over USD $10 mill, you should take in enough to keep the ETF going without heavy cost to the issuer if the annual expense ratio stays under .5%.
Dead ETFs Walking [View article]
As long as you have assets over USD $10 mill, you should take in enough to keep the ETF going without heavy cost to the issuer if the annual expense ratio stays under .5%.