Absolutely C.
If there is any possibility that this person or entity may become a long term client of yours - as in site building, creating ads, consulting - then there is no reason whatsoever to overcharge for added domains.
Tell them up front what the cost will be and offer to register and push to their account.
At some point in time, this client may find out that you overcharged them and that may not set well.
I use my own example of a friend/client who has had business online since 1995 (or so). For all these years he has paid for hosting, domain renewal, and all the other services through someone like Register.com. Somewhere along the way he was talked into registering all these others to "protect him".
When we started working together and talking about site and site development, he had no clue about tracking, adding adsense or other affiliates, analytics, or seo. When I explained this to him and showed him some of the options he could not believe what he had been paying for and what he was missing out on (as well as overpayment).
Fortunately for him, his business (brick and mortar as well as online) had been and is very successful thanks to a highly searched keyword niche domain.
Now I have his business and friendship and respect.
---------- Post added at 01:48 PM ---------- Previous post was at 01:47 PM ----------
Ok, I see you have provided an update.
A decent price for your services and time.