Zone 2 and 3 aren't very "outer" in terms of areas, so you're likely to find it more expensive than someone in, say, Zone 5 (like me).
What you pay is going to depend very much on where you choose to send your kids. We pay £4.50 for care with a registered child minder who does early years education, but we're also looking at a private nursery school at £2,965 a term (if I choose to go to work out of the house full time) or £1,780 a term for three full days (8:30 to 3:15). Those costs take into account the 15 free hours a week. If we go that route, the childminder will still have to pick him up from school and watch him until I get home, likely around 6 or 7 (yeah, don't live out in Zone 5) at £4.50 an hour, which is far cheaper than using the school's crèche (which is only open until 6).
It might be a good idea to check out local parenting sites for ideas of costs near where you plan on living. I found netmums to be an immense help when going over my options for childcare and it's very localised.