Alterations Prices

I think this is going to depend on what kind of straps they are - are they embellished? Plain? How are they attached to the dress?

For example - a dress like this, the straps are kind of incorporated into the look already and would be very difficult to alter thinner:

But a dress like this one..

the straps are almost separate pieces as it is and would be a lot easier.

Your best bet is to take it/show it to a few alterations people and get some quotes.

That mermaid dress is bomb