AFAIK, if you are buying with a credit card, DMM will just charge you an amount in your currency equivalent to the yen at the time. 1 DMM point is the same as 1 yen, so with a currency conversion calculator, you can just convert the amount you are buying in yen to your currency to get the estimate amount you will pay. It doesn't seem to discriminate between local and foreign cards, at least from what I have read on Himeuta. Your bank may charge you with an extra fee for foreign transactions though.
I have a virtual prepaid CC (a MasterCard) linked to my DMM account and use the credit card option, and going through the payment log on my virtual CC's app, I haven't seen any over-the-top charges from DMM, only an additional $1 charge that is refunded back to me once payment is cleared the next day. The company that receives my payments is "DMM.com" / "DMG," and I always get an e-mail from DMM confirming my purchase.