I would just right align:

#set text(font: "Segoe UI")

#table(
   align: right,
   columns: 2,
   [*category*], [*cost*],
   [widgets], [#"42.00"],
   [gadgets], [#"1.10"],
   [thingies], [#"890.00"]
)

or you could do something like this:

align: (col, row) => if col == 1 and row >= 1 { right } else { center },

if you really want to nitpick, you can also mess with pad:

https://typst.app/docs/reference/layout/pad

align: decimal.dot or align: decimal.comma would be great.

Ideally also an arbitrary alignment mark align: "=" so that the equations or numbers do not need to be modified.

\begin{align} 
2x - 5y &= 8 \\ 
3x   9y &= 4
\end{align}

I think this should be a built-in feature. It's fairly common in some end-uses to have differing numbers of digits both before and after the point, as you move down the columns of characteristics from, say, small to large product models. In certain page layout apps I'd just centre align then pad the whole column with figure-width spaces, assuming a font with fixed-width digits/decimal point. But a built-in markup for decimal (or arbitrary character) alignment makes things easier. As indeed do figure spaces.

I think, by default, the decimal delimiter that's used for alignment should depend on the context language.

Also, it should be compatible with existing cell alignments:

|left num     | center num  |    right num|
|_____________|_____________|_____________|
|100.359      |   100.359   |      100.359|
| 99.23       |    99.23    |       99.23 |
| 21.445      |    21.445   |       21.445|

Non-number symbols should be aligned on their own axis. Not sure how to handle " " and "-". Maybe that should be configurable:

|       prefix|       suffix|                      sign?|
|_____________|_____________|___________________________|
|    > 100.359|   100.359 mm|     -100.359|     -100.359|
|    ≥  99.23 |    99.23  µm|      -99.23 |     - 99.23 |
|    ~  21.445|    21.445 nm|      -21.445|     - 21.445|