Hi, sorry can't comment on costs but my DD was in childcare for 2 days a week from the time she was 18 months until she went to school and she did not do a separate 3 year old or 4 year old kinder program. At the childcare centre they had a kinder teacher in the 4 year old room and she was moved in there as soon as she turned 4 and had 18 months in there before going to school.
3 and 4 year old kinder are sort of about putting some structure into their day and their learning to prepare them for school so it does not really matter where they get it, in my opinion.
good luck