Any parent will tell you what I'm about to tell you: There are times that I'd give my life for C Batteries. The pack-n-play vibrates on a C cell. The swing moves on a C cell. The vibrating bouncy chair runs on a C cell.
I heard someone say it before I was a dad and now I believe it: If Menards charged $50.00 for just one C cell battery - I'd pay it.
Then make sure it runs on your own proprietary battery. Make that sucker last only like 3 weeks. Make me go out and buy them for $50 a pop.
Guess what? If the swing works? You'll have me as a customer. No matter what it costs.