Personally I wouldn't charge during the closed beta because the insights you will gain from your first customers is worth more than money.
That being said, if you need to keep the roof over your head, let a few people try the product for a short amount of time, and ask them how much they would be willing to pay. You can fine tune the amount as you add more customers and get more feedback.
That being said, if you need to keep the roof over your head, let a few people try the product for a short amount of time, and ask them how much they would be willing to pay. You can fine tune the amount as you add more customers and get more feedback.