Last winter we have one project were we need clients font. We asks that from print agency but they have only own licenses, so we had to buy font. Of course we billed that from client.
I don't know about custom created fonts. If you have to buy it, you have to billed it too...
It's part of the client's expense (whether you buy it or not). So, it's up to them whether they want to give it to you or whether they want you to buy it and put it on their tab. Why? It's what's needed to get their job produced. Same goes with any stock imagery, any printing costs that would have to be done, etc. If their general (non-interactive) agency has the font, it might be easier requesting it from them.