Is it possible to customize the Task cost calculation?

Christophe,
First, I suggest you read this Wiki article on data types in Project:
https://web.archive.org/web/20200720083203/https://social.technet.microsoft.com/wiki/contents/articles/31991.ms-project-data-types-task-resource-assignment.aspx

Then you might also want to read the article referenced in the above concerning extra fields:
https://web.archive.org/web/20200722225529/https://social.technet.microsoft.com/wiki/contents/articles/32051.ms-project-extra-fields-in-views.aspx

Those articles will explain why your formulas don't seem to do what you want. However, based on your description, I don't see a need for custom cost fields at all. Instead of creating separate custom fields for funding types and resource types, why not combine the two. For example, you could have an EC work resource, an EC material resource, an EC cost resource, a Regional work resource, Regional material resource, a Regional cost resource, and so forth for each funding entity. Set up each resource type with the appropriate rate based on how they fund. You can even set up a time based cost escalation and/or up to 5 separate cost rates for each resource via resource cost rate tables A through E. This is accessible through the Cost tab on the Resource Information window. Varying cost rate tables for each resource are applied via the Cost Rate Table field at assignment level on either the Task Usage view or the Resource Usage view.

Hope this helps.
John