1 - Yes, its ok to charge for dev costs && keep the resultant product for your own use and to white-label to another.

2 - Its ok to do the "powered-by" thing. Best to add it and then put a config option for the customer to turn it off fairly easily.

3 - What to charge? Well if you have no idea what the wallet size of your customer is, charge about 2/3 of what you think it would cost him to write it from scratch (what it might cost him, not you) and then provide for hourly/daily rates for consulting work for mods and support.