Price Set with a conditional 'Other' field
You can create an option in the event fees for 'Other' with a price value 0. Then create another price field with the type 'Text / Numeric Quantity' and a price of 1. Some other things you may want to consider with this method:


Hide prices in the field setup and put them in the line item descriptions. This avoids the user seeing the $0 amount on the 'Other' option.
In the 'Other' line item description you may want to add something like 'Enter amount below'
With some simple jQuery you can hide the other amount field unless the 'Other' option is selected.


Hope this helps...