In a typical demat account there are a variety of charges that you will incur. Firstly, most DPs will charge you an account opening charge. Of course, there are some brokers who will waive this cost but more often than not there will be an account opening cost for you. Secondly, you will be debited an account maintenance charge (AMC) to your account on an annual basis. This is irrespective of whether you transaction the account or not.
Normally, as your number of transactions increase, these AMC charges will come down. Thirdly, when you open an online trading account, you will have to give a power of attorney to your DP to debit your demat account based on your instructions in the online trading account. This requires a stamp paper agreement to be executed and that cost has to be borne by the account holder. Fourthly, there is a charge on each debit from your demat account. In the case of most brokers it is a fixed fee per debit.
Lastly, there will be miscellaneous charges like physical statement request charges, physical transaction statement request charges and penal charges when your DIS is not honoured etc. All these charges when put together can add up to quite a bit. These all are for just operating your existing demat account with shares bought I demat mode. In case you are holding some shares in physical form then you also need to send the same through your for dematerialization. For that you fill up a demat requisition form (DRF) and submit to your DP with required details. There is an additional cost for dematerializing physical certificates on a per folio basis.