If your ghs is too low on cex.io the maintenance cost will impact you, they offer the low cost on the front end, then hit you up later on for operating costs. If you "own" enough gh/s, the maintenance fee will won't be as large of a percentage against your mining revenue.
OK, maybe I'm misunderstanding here, but isn't the maintenance cost a fixed percentage? According to their web page it's $0.18 per GHS, so your fee as a percentage of GHS does not change. Unless I'm missing something.
I can't answer the question, however I used the service a few months ago, and don't recall the service charge being an issue.
I had another go last week, and was surprised to see so many appearing.
Maybe the slowness in finding blocks just gives the impression that there are more service charges. It's a shame they don't put it on blocks found. I bought around 650 gh/s, and the first action on my account was a debit (no blocks found in 2 hours). But anyway, less bitcoin mined means service charges become very much an issue, since you have no way of making up for them every 10 minutes or so.
I can't see where the profit is at the moment, which is why I sold my gh/s on the same day, at a loss.
My experience a few months ago was better, when the difficulty was low.