I started my quilting business focusing on making t-shirt quilts-start to finish. I charged 11cents/sq inch--including the initial cutting, the stablizer, the second cutting, the piecing (and hours of design ahead of time if all shirts are not same size), the quilting, the squaring and binding. I think due to increased costs of fabric and stabilizer, that a more reasonable price today would be 13cents/sq. inch.
Do not start cutting all those shirts before you give her a cost estimate and time estimate too.(when you can have it finished by)
One thing I did to avoid the whole "include every single shirt" was to get a flannel backed plastic table cloth and draw the different sizes (lap, twin, queen) and folding shirts to the logo size you'd use, lay them out with her. Most figure out real quick that every shirt will not fit and costs become real apparent, too.
Good luck--after 30 t-shirt quilts (and one donated one left for the local library, maybe future one's for own grandkids) I no longer do them for customers--not particularly fun, and people really don't want to pay what is needed to be profitable.