Did you not think to discuss pricing for extras prior to the time of them being done? Could it be that the sparks got so fed up with you changing things about that they have hammered you the only way they know in order that it may teach you to be more thoughtful in the first place, no point in producing plans if you don't intend to stick to them, may as well have a blank piece of paper. I recently did a refurb for a very nice chap, it started as alter a few sockets to being a 98% rewire. Nothing was agreed for a fixed price, how could it the chap would change things almost daily so I charged what I considered to be a very reasonable day rate, this was irrespective if I was on site a whole day or half day. Was the chap annoyed? He may of been, but then he appreciated all the effort and hard work involved by me for him when he changed something, now did I get fed up, almost definitely as I was doing this mainly at weekends and everytime I thought I would be finished something changed, could I have charged him more? Quite possibly, but all things considered I earnt what I was happy with and he paid it regardless of his feeling.
Lesson to be learnt, if money or paying is an issue then make sure you are fully aware of any costs at the outset.