Increasing the width panel in the middle might be an option, although it would depend on how much cleavage you were happy to show doing that. If you didn't want to show too much cleavage then doubling or tripling up the tulle panel so it is hardly see-through might work. However, widening the central panel might then distort the line of the dress, as it currently sits in a neat "V" from the shoulders to the waist. If you widen the sheer panel there, it forces the "V" wider which could spoil the line of the dress above to the shoulders. As it's the main focal point it would be very obvious.
The other option is taking some of the fabric from another part of the dress (eg hem, width) and creating an extra panel under each arm each side of the side seams.
Either way, whatever you might do depends on how your chest sits in the dress and where any extra allowance would be best placed. You'd need to try it on for a dressmaker to establish that. Some dresses won't fit some busts no matter what, as the design won't allow for it without ruining the line of the dress.
Presuming the dress is lined, the dress bodice (top section) will need opening up for both options and extra panels added from the inside, then it will need closing professionally again. This is where the expense would come in as it's time consuming and difficult to do. People imagine you can just cut the central panel off and sew a new wider panel on behind (and maybe you could) but this won't necessarily look great (nor would it be something a professional seamstress would be happy to do).
I'm not a seamstress but I did once have a dress made bigger at the bust. It was a strapless prom dress I'd fallen in love with, originally expensive but in the sale and a size too small at the bust. In that case, adding a stretchy elastic panel under the arms (my idea! 
) was quickly discounted as a very amateur way of going about things and instead the seamstress added an extra tiny panel either side of the central back zip, a slim triangle either side running down to meet at a point at the waist. I don't think that would work on your dress though, as it's already a "V" shape (mine was straight across, being a bandeau bodice).
My seamstress was very very talented and couture trained, so my alteration looked great, like part of the shaping/design. I learned a lot from her re what goes into altering a dress. She wasn't cheap though!
Looking back I'd probably have chosen another dress instead of all the expense and hassle, but I was happy with it on the night.