In what sense did God 'redeem' Israel from Egypt, and was a price paid?

No, there was no price paid. It was simply that the Israelites were no longer going to be under the gods of Egypt but under Yahweh, and to accomplish that they had to be taken out of Egypt; it was a religious matter, not a payment. Pharaoh received nothing; in fact he ended up paying a great price through the ten plagues, the last of which cost him every firstborn in Egypt. So if anyone paid a price, it was Pharaoh, not God. 'Redeem' here means to rescue someone from oppression, as God rescued Israel from Pharaoh's harsh bondage of making bricks; the problem was that they were multiplying so rapidly, up to some three million people, that they became a burden Pharaoh would not admit he could not handle, so he made it harsher. But they were mature enough as a people for God to take them out and start a society, and the irony is that they had already begun rebelling against God back in Egypt, and only two of that three million ever reached the land of Canaan.