You should also ask yourself how is the client going to benefit from their ownership. You/your company owns the copyrights and there are a few options here for rights;
1. Full granting of rights - this gives them a lot of room but should be the most expensive.
2. Usage rights - specific rights for specific uses and payment is based on that use or number of uses or number of times the images will be used. (Ie. We are going to use it on a printed map and print 100,000 of them) but you maintain complete ownership.
3. Limited time use - used for a limited number of years but normally it’s agree to specific use and you maintain complete rights.
It’s all a negotiation and you might have to consider, how easy would it be for the client to replace you, either through buying the equipment or hiring someone else who’s more willing.
But this is very normal for copyrights and I would guess your client completely understands this, hence them wanting the rights.