An alteration to an instrument is something you cannot attach a standard price to. For this these interventions are too dependent on their nature and extent and also on the instrument in question.
This means that such alterations can only be done by mutual consent and must be rated from case to case. The finish plays a role as well.
Changing a right thumb hook is a comparatively small job which can offer a lot of ease. A lot more complicated is the altering of the left thumb position or changing the height of the left hand palm keys, although ergonomically this can make a great difference.
Adding a ring to the tonehole of a single too high pitched a tone is done in a jiffy. Enlarging a tonehole is possible as well, but is a lot more complicated. In practice this will be done only in the case of very serious faults.
Seldom something needs to be added to the mechanism of a saxophone, although I've made a couple of C#\G# linkages. Adding a C#-mechanism is quite a lot of work. Downsizing register holes on the other hand is a piece of cake which provides possibly a substantial gain in intonation.