Hello. This is very workable. If you setup the custom fields correctly, it will calculate the new price and show it on the confirmation page... then the user will be given payment options as you have configured.
I would suggest a radio button or dropdown that gives three options:
Saturday Only|Sunday Only|Both Days
Assign the appropriate fee values which would look like this:
50|50|100
Be sure to fill in the Selection Limits field. If there is no limit, just enter:
0|0|0
In this case, I don't think you'd need to give an actual limit. The capacity check would handle this for you... if needed.
Then create another custom field like:
Will you have a guest at Saturday's Dinner? Yes|No
If they select Yes, then the $75 can be added.
This is standard DT Register functionality
Take a few minutes and watch our custom fields video tutorial to see all of this if you need further assistance. It shows a handful of examples doing things like this. Thanks.