There might be some patterns (period and reproduction) for hoop dresses that you can take to a dressmaker/seasmstress. They can alter the pattern to fit you, and finish it how you would like. I will say, that if it is a complicated style and you choose luxurious fabrics, that would not be inexpensive. It would not be like a simple shift dress that could be whipped up quick, so plenty of lead time would be needed depending on the bodice too. There may be reproductions out there also.
You might be surprised when you do some hunting, you may find other authentic ones under your budget. They might not have all the bells and whistles, or might have to be partially restored, but would bring you under budget. The better condition something is in plus the better quality, to begin with the higher the price goes when comparing it to other similar dresses.
Since you don't need it til next fall (2006 you mean, right?). If it is something you truly wanted, would the shopowner/seller be willing to work out a layaway with you if you put a certain chunk down? Sometimes if you truly love something, there is just no substitute.