Nice, but what do you mean by "pop in and surprise her"? Will she be in Vegas prior to your arrival?
Cost? Spend as much as you can to make it special and memorable. I have no idea. Consult several hotel casino wedding staff to find out what is available and for what cost. But hurry. April 20 is likely already booked up if using their services.
Or maybe just surprise her with dinner at the [expensive] high in the sky restaurant at Paris or elsewhere. Reserve it NOW, discuss it with someone who can insure the best table in the place.
Since it is your anniversary it will be a challenge to actually surprise her unless you do something like this that is beyond the norm.
What places (hotel or the dam or museum or whatever) does she love? Something she always wanted to see or do?
Don't forget the wedding chapels. Call around and ask what they can do to make a re-doing of the vows, etc. fun and special. A favorite "performer" for the 'pastor' or whatever the person would be called
Congrats on 40 years. HURRY!!!
Candy