Hi admemo,
over the time several reports of this problem have reached me. Possible causes are:
1) Name conflict
My choice of the class name “Options” has been very unfortunate, because it is likely to conflict with another plugin that uses the same class name. If this is what is happening disabling all other plugins would fix the problem.
2) Incomplete WordPress update
If you don’t follow the correct WordPress updating process the outcome can become unpredictable. I know of one user that has not deleted wp-admin and wp-include folders before refreshing all files to the most recent versions. Surprisingly she got the error you have mentioned as a consequence of that.
3) Are you sure you are using PHP 5.2 or later? Earlier WordPress versions did support PHP4, but my plugin did not.
By the way, it’s been a while now since I published price-calc. I’m currently working on a new, completely refurbished successor called Jazzy Forms. Its configuration will be less of a pain.
