Some people dont like handing out keys, for various reasons, you say yourself the job is a nightmare, although you dont explain why

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Any extra hassle, that the job puts you through, for whatever reason, means an opportunity to charge more than normal, as a means of compensating you for the extra grief, if you like, much better, this way, than to walk away from a regular contract.
If the client, queries this sudden price increase, be honest and professional with them, and explain, that their own insistance on doing things their way, increases your own difficultly in servicing this contract, hence the extra cost, clients very rarely, mean to cause offence, mostly its just a lack of technical knowledge and communication, try not to let it get you down, or take it too personally, regards,
Rob