From your description it sounds like this is a commercial system where one could assume a certain level of commitment and some serious resource availability. (Correct me if/where I'm wrong.)

I here assume that you have a backend database for prices, 'gizmos', etc. I guess it's some kind of SQL database. If so, I would implement the rules system in the backend database.

You build your frontend application to understand possible generic rules like 'volume discount percentage' and 'volume discount threshold'.

Then you let your co-worker manipulate the rules in the database, using any tool s/he's comfortable with. Either a specific tool built by you or a generic database manipulation tool, all depending on your co-workers skills.

So your co-worker could make sure that for 'gizmo', the client must by 30 units before having a volume discount, but for 'thing' the client must buy 50 units. Furthermore, the volume discount for 'gizmo' is 20% whereas for 'thing' it's 23%. Etc.

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Everything went worng, just as foreseen.