I think that for cornrows anything over $50 is too much.
I think it is best to go look at their work in person before you decide to give them your service. Pictures don't always do the job. You should see for yourself how tight the edges look.
I am less of a question asker and more of a Iron Fist dictator.
If you have baby hair around your edges, tell them that you don't want them to catch that and braid it in. If they do, it will likely cause damage.
Do you like the kind of cornrows that start small near the front and get fatter? Or are you OK with the kind that have the knot at the front?
Make sure you know whether they can achieve the look that you want.
Also as they are braiding your hair, Do not be afraid to speak up and tell them immediately if something is pulling too tight. If you tell them right away then they can adjust the tension rather than having to do the entire row over again.
If you are going to have the extensions curly at the ends. The usual method is to roll the extension hair onto a roller and dip it in boiling water.
This is OK but you absolutely cannot allow your hair to be dipped in the hot water. So...they have to be aware of where your hair stops within the braid and only dip the extended part.
And find out if you need to bring your own hair or if they will provide the hair.