My starting shirt was smocked but yours doesn't need to be. It's easy to do on your own. Just follow along to read how.
What you need:
coordinating scrap fabric
ribbon or lace or braided fabric for straps
chalk/pen/marking device (if you are using something permanent then I suggest drawing on the inside of your shirt instead of the outside also DO NOT USE A MARKER. It will bleed through the fibers and not look pretty.)
The amount of scrap fabric you need depends on what size jumper dress you are wanting to make and how big the shirt you are starting with is. If the shirt you are working from is large enough you may not need any extra fabric at all unless you just want the contrast.
This can cost as little or as much as you want. You could use another shirt for you scrap fabric even if you wanted and if you have the shirts already or someone gives them to you then you could do this for free assuming you already have your sewing supplies.