It's possible to convert a valid string representation of a number to a Double but if the string doesn't represent a number then what conversion could be made? If there's a chance that the string may be invalid then use the TryParse method the type you're converting to:
vb Code:
Dim price As Decimal
If Decimal.TryParse(txtPrice.Text, price) Then
'The conversion was successful and the "price" variable contains the number.
Else
'The conversion failed.
End If
Note that I used Decimal instead of Double. It will work with Double too but it is generally preferable to use Decimal for currency values as there is no risk of round-off errors.