Is this an issue you can reproduce?
is the cart static or dynamic?
if dynamic, make sure that all of your prices are valid numbers, without the dollar sign in the database.
this error usually occurs if the amount for an item is not a number. if the price passed to authorize.net included the currency symbol "$3.02" it will cause this error.
it could also be caused if you have discounts applied that cause the total of the cart to be less than 0.01 cents.