can we obtain a Netsuite sandbox account as development company for development purpose ? if yes how much does it cost ?

Yes this is fairly common to have more than one sandbox account and use one for development. You'd need to talk to your NetSuite account manager about cost, but I think a few thousand dollars/year is what you can expect

Thanks @Steven Marquez. We don't have any production account though , we just need standalone sandbox or a developer account ,Do you know how can we get one ?