I'm going to go out on a limb here and suggest you're doing it wrong :)
I'll start with the statement:
> ... all in one column
That's a recipe for disaster as far as spreadsheets are concerned.
It is impossible to add "Rent 1000" and "Lawn 40" and come up with any sane answer.
At the very least these should be in two columns, for example 'Description' and 'Cost':
Now it's easy to add up the values.
Ideally, the Category should be in a separate column, too. That will make the rest of your ask a cinch.
For completeness, you might also add a date column - that makes it trivial to get a breakdown by month which will let you see trends, outliers, etc.
Numbers will do all the heavy lifting for you, but you have to play by its rules and give it the data to work with.
For an example, here's an extension of the above table with those extras added:
It doesn't look like much yet, but now you have two options - Categorization, and Pivot Table.
With Categorization you tell Numbers how you want to logically group the data, and it's controlled via the Organize -> Categories Inspector:
For example, if you want to break down the data by Category, tell Numbers that's what you want:
almost instant gratification.
Note there's no subtotal by default - to add this, just click on the Cost field in any header row and tell Numbers what summary you want to see - in this case, SUM:
Or maybe you want a month-centric view to see your expenses by month - just change the Category to the Date field:
You can even add multiple categorizations if you want to break out by category and date, for example.
So many ways to slice and dice your data.
The nice thing about Categories is that it's dynamic - you can switch it on and off at will, and it always shows real-time data based on whatever's in your table. Typically you might turn it off when you're entering data, then turn it on to see how it breaks down.
The alternative model is a Pivot Table. This add an extra level of functionality (and complexity, to be fair), but allows for a two-dimensional view of your data with any combination of category, description and cost in either rows or columns.
For example, if I want a table showing category by month, I might set it up like:
Or maybe I want to see the individual expenses, so I add the Description field to the Rows:
There are pros and cons to each approach. Pivot Tables will automatically add subtotals for each row/column, and sometimes it's easier for two dimensional data (e.g. categories and dates, such as we have here), but Pivot Tables work on a snapshot of your data - they don't update automtically due to the overhead of working out all the permutations and combinations, so you need to click the Refresh icon (at the top of the Inspector) to reflect the latest data in your main table.
