Yes, it will work!

Designed properly, such design allows:
Register project's detailed phases and their budgets/costs monthly (I advice to use month number in format yyyymm);
Register project's start and end at any month;
Register projects of any length (from single month to several years);
In case this is needed, registering both planned budget and real costs spent;
Design monthly and yearly reports for single project, and as total of all projects in report period;
Design reports for totals of selected project (from start to end);
etc.

Based on your post, I assume the main data entry table will be, where planned (and realized) phases of project are registered (E.g. Month, Project, Phase, [Type], Amount, ...). (Type is optional - in case you want enter both planned and realized phases.)