Fair fee for 3d files

Recently I worked on a packaging project that involved series of 3d illustrations and an animation. After I completed the illustrations, the client decided that they would like to take over the animation part and asked for all 3d files.
They understand that there is a cost involved as the payment agreement was for the final deliverables only. So what would I base the price on for these files? time it took to create the 3d model? percentage of the final fee paid so far? I never had such a request and I’m not sure what would be a fair fee for this.
Thanks is advance!

I would charge for your modeling time, time required to generate the illustrations, plus any time spent for communication and quoting.
I would be clear that any follow-up from the client tied to this project will be subject to additional fees.
( file conversion, color tweaks, material maps, etc.)

Whatever you decide, don’t sell yoursefl short: if the bulk of the time was spent modeling and generating illustrations and the final animation is a turntable sequence, it’s fair to assume that your labor costs thus far should eat up ~80% of your budget.

I would be also careful as the 3d models qualify as a fully designed/developed part.

Make them pay 90% of the total project (take off % for deposit paid etc…)