Will they be to make my dress bigger?

I bought a dress that is a size 8. I am between sizes 8 and 10 (working out like hell as well) and when I tried on the dress i was able to fit 90% of the dress. the only issues were the side zipper which came up half way (I think due to my chest) and I can't lift my arms all the way(half way up, but not free range). Took it to the cleaners this morning and he was able to find some extra material in the back (was pleated in the back) so he believes that he can open up the back to add about a 1/2 inch. I think the can. what do you think and how much do you think it will cost (I hope no more that $45)