Strange problem. I've just added a new price field to the price set on my membership renewal form, and whenever that price field is used, the link to PayPal fails with this error on the PayPal side:
"The link you have used to enter the PayPal system contains an incorrectly formatted item amount."
Any submissions made where there is no data entered for this field work correctly.
I've tried recreating the price field, changing the price, and changing the order of the fields. But the result is the same: the confirmation pages show correct totals, but if that one line item is included, PayPal throws this error.