even if the floor is rock solid, I would still use 12mm.....your screws can get a countersink and wont burst through the ply

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even if the floor was rock solid.....i would still use 12mm (min) ply,
it helps to get a good countersink on your screws without letting them burst through the ply

as for the price depends where you are...expenses...how long it will take you...for brick bond you need a dead flat floor, or it will take longer.....
just be fair to yourself and the client and you will get repeat work and reccomendations = more money!