This is really a custom solution and I won't be able to give very extensive advice but I think you could do this all server side by having the checkboxes on the event registration page.
This page would need to be an asp page or post to one that you have control of. On the page that the form posts to you can just do some simple checks for each of the three checkboxes. If any one of them is checked then an extra amount can be added to an amount variable.
You would then just use this variable in the email message to indicate the price.
To get it to show in the browser as the user makes the selection you would need to use some client side js for this. You can do something similar where you have an amount variable and accumulate the value based on the checkboxes. You can then write this value to a div that displays the price.
This is just a high level overview and is not something that would be supported since it is custom development. I hope this gives you a better idea of how something like this can be achieved.
