There are several possible approaches. One that you may want to consider for contexts other than your dashboard layout is to set up a list view summary report with a Sub Summary layout part "when sorted by" your category field and with the category and summary fields placed in your sub summary part. Such a report can list the indivdual records or you can remove the body part and just list the categories and their totals. This is simple and flexible, but probably not what you want for a dashboard layout where you likely have many other layout objects not part of these totals.
For the following alternative methods that should work for a dashboard layout, I recommend setting up a table where you have one record for each category. You can define your value list to list values from this table and you can add/remove/change your categories by editing the records in this table.
If you have FileMaker 12 and aren't afraid to try your hand at using SQL, the ExecuteSQL() function can be used: FMP 12 Tip: Summary Recaps (Portal Subtotals)
Without using SQL, you could set up a portal to your table of categories and use a relationship from that table to your table of expenses that match by the category field to compute a total for each category.
With both approaches, some thought should be given to how you will set up criteria that further filter down the records used to compute these totals so that they compute totals for specific date ranges such as all entries for the current year, the current month and/or the current quarter. Whith Execute SQL, this can be specified in a WHERE clause. With a portal to the categories table, a portal filter or a global match field can be set up to get the same "filtered result".