Although it wouldn't be a professional, perfect fix, if this is a strapless gown that just needs a bit of "tightening" at the top, you might ask one of your friends (or their mothers) who know how to use a sewing machine if she could take a look at the dress and see if she could re-sew the side seam on either side of the bodice, by making it as tight as you need it on top and tapering it down to the current waist. I haven't seen the dress, but that might work, depending on how it is constructed and if there is "boning" (a rigid plastic piece you can feel through the fabric) involved.
As for straps, I like the idea of the jeweled straps another poster suggested. Look around a fabric store for ideas. You may find a number of creative alternatives. Ask for help, as the people who work there probably all sew.
These ideas might help, if your goal is to keep the cost down as low as is humanly possible. Good luck!