My dress costed $830 and I am paying $225 to have it altered. So, it will be $1055 total when I am done. I personally think that is a lot of money to spend on a dress that I am wearing for 1 day (and TTD the next day), but my max was $1500 and I am below that, so I am happy. I could have found something cheaper I am sure, but I love this dress, so I did not care. I am having a few things changed, but nothing major. I think the back is important, but not the most important thing. Personally, I think if you don't love all of it, than maybe you should keep looking. $800 is a lot to spend to alter just the back of the dress, but that is just my opinion.
I am not sure if that helped, but I hope it did somewhat.