Should it be:
What's the cost for each available size?
What's the cost for each size available?
How much is the cost for these things?
How much is the cost of these things?
Are both okay?
There's no difference between 'available size' and 'size available'.
Don't say 'How much is the cost?'
Say 'What's the cost of each size?' or 'How much are these things?'