In programming, comparing strings does not work the same way as comparing numbers. If you wish to compare the string values as if they were numbers, you will need to convert them to numbers first, then compare them.
The same applies in your case. For the sake of handling the data by its type, RadGrid has built-in columns such as Numeric column, Date, Image, Link and more: RadGrid - Column Types.
In order to make the filter handle the data accord to its type, you can try using GridNumericColumn, or setting the DataType property of the column to System.Decimal. In some cases, you may also want to bind decimal type of data to the column.
I hope this will help resolve the issue.
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