We charge Rs.50 + Service Tax per month towards platform charge. So for 12 months, including service tax at 15%, you would be paying 57.5 * 12 = 690/-. Besides this, there is an AMC on your DEMAT account which is at Rs.300/- per year. So totally you would be paying 690 + 300 = 990/-
But please note, your DEMAT will not only safeguard your MF units, but will also safeguard and hold your shares, Bonds, ETF etc. So it is a single vault for all your assets held in electronic form. I guess 300 per year is still way cheaper than your bank locker
Besides, 690/- against the MF platform is a fixed cost - against this fee you can invest any amount across any number of funds. Unlike the traditional 'Regular MF investment', there is no % commissions here. So your savings tend to be way higher when you invest via Coin (or any direct mutual fund) which as you may realize translates to a better return on your investment.