What type of shotgun is this (Browning, Aya, Baikal...?)
Is it chrome bored (bookoo $). What type of choke tubes you want also makes a difference. How thick the barrels are at the existing choke also plays a role. Depending on the above, the same can be said about opening the chokes. Contact a reputable 'smith who has knowledge of your type of shotgun and ask for a quote.