You can utilize your existing equipment with some modifications to be as low-cost as you can get.
Secure FTP Server (instructions – http://www.digitalmediaminute.com/article/1487/setting-up-a-sftp-server-on-windows)
Dynamic DNS account (free)
Router supporting DDNS (DD-WRT – open source firewall for most home routers) – free
FileZilla client – http://filezilla-project.org/download.php?type=client
1) Create an account on one of the dynamic dns providers (DynDns)
2) Open ports (22) on firewall inbound to your SFTP Server
3) Have Rule 2 above pointing to internal IP of your SFTP Server