If you go with the $40 plan, then after you run through your 100 minutes, additional minutes are charged at $0.25 per minute. Pricing for additional sent texts is also $0.25 per. They do not add an extra $40 or extra $15 or whatever. Additional data is charged at $25 per 100 MB.

If you know you are going to use more than 100 MB data and more than 100 minutes/texts, then there is an $85 plan that gives you 250 MB data and 250 minutes/250 sent texts ($35 for the minutes/texts on top of the $50 for 250 MB data). Details can be found by clicking the "need more data, minutes, or texts?" link on the Verizon global roaming plans page. I don't think it is possible to add this bundle without calling in, or at least I couldn't figure out how to do it on the website.

The Verizon customer service agent I talked to said that it is not possible to do a plan that gives you 250 MB data and only 100 minutes (which is what I needed), and it turned out to be less expensive just to buy an additional 100 MB of data than to buy the more expensive 250 MB/250 min/250 texts plan.