concurrent users hitting web app

I am a business analyst and db designer who has the seed of an idea for a web/mobile app. I am doing a feasibility study on a web/mobile fitness app. I've talked to a couple of my colleagues about the idea and have hit a brick wall on dealing with a potential challenge related to costs of a app that might have a large number of users hitting the site concurrently. The question came up because I had planned to store user stats and profiles on the website. Data would be uploaded to the "mothership" whenever a user has new data to add to their profile for dash boarding. My colleagues have told me that licencing for whatever I use for data storage is going to cost me a fortune and that profiles should be maintained on the users mobile device. Is this true? This is a startup project and I am not sure I want to finance a big dollar system until we experience growth in memberships. I am an absolute neophyte with this so any help, advice or direction would be greatly appreciated. Many thanks in advance! Rob