Most service providers won't hide such costs, and will clearly state them in their pricing summaries. When weighing costs be aware of these things. Some services even charge for support. Ask yourself how likely it is that your list will ever incur the extra charges. Finally, hosting services have different pricing systems.
Services charge by a variety of criteria.
Some of the most common include list size and list volume. Some divide categories into discussion lists and announcement lists. Some services charge monthly, some quarterly. Some charge by messages delivered. This can make it hard to accurately compare services, so keep good track of basic costs for easy comparison.