That implies your users are uploading files. Certainly not the case for all startups, but yes, this could certainly be an additional overhead. Still, compared to running and configuring a VPS, thats nothing.
Also, if you're handling user uploaded content you shouldn't just be writing it to local disk. You're going to have to figure out how to do off-instance blob storage at some point if you scale beyond a single server or want any redundancy. S3 is just about as good as it gets for blob storage today.
