You can get 8 rechargeable AA's from amazon for $12, or $1.44 each. Non-rechargeables are $0.78 each. If a rechargeable battery prevents you from having to buy a second set of non-rechargeables then it's paid for itself.
If you want, you can just put rechargeable batteries in their most commonly used toys. Or better yet, teach your kids how to replace the and charge the batteries themselves. A 4 or 5 year old should be able to handle that without issue. And it means you aren't doing everything for your kids, which means they learn some self-sufficiency