Be sure to first understand that different people/organizations all have different definitions on what they mean by rounding. Once you are clear on what you need, you can use Alteryx to round your numbers.
Example 1:
To round a number to a specific place, first divide the number by that place value, then use CEIL to round the result up, and finally multiply back by the place value.
324 to 400
CEIL([Number] / 100) * 100
Example 2:
Ceiling and floor just go to the nearest integer. The ROUND(x, multiple) function will allow you to specify the level of precision on which to round, very similar to Excel. But you can't specify the direction, so in your example, if you did ROUND(3425.123,1000) it would return 3000.
Example 3: Please see the attached v10.0 workflow, which is more dynamic.
The text input identifies the mapping for the conversion, joined with the length of the source input - then an expression is applied in the Formula Tool.
The final output.
More on rounding.