When the Nice 2Checkout Button plugin for Joomla was developed, variable pricing was not an option. The 2Checkout API mandated that all product information, including price, be stored on the 2Checkout site. This made it impossible to allow buyers/donors to input their own price amount prior to checkout. I will research 2Checkout API changes and see if this has changed. If it has we'll do our best to include any changes on the next Nice 2Checkout Button plugin release.
In the meantime, please consider using the Nice PayPal Button plugin. The plugin and PayPal provide the kind of flexibility that you are looking for.http://trinitronic.com/index.php/Downloads/nice-paypal-button-plugin.html
~ Best regards