Depending on what company you host your web site with, you might already have the capability to manage your own simple FTP site. Most budget hosting accounts use "cPanel" or other easy-to-use controls that let you create ftp sites and users, so I'd first check with whatever hosting company you're already giving your business.
If you're looking to switch hosting companies, I've been using JaguarPC for hosting my personal sites for ages for ages, and have always been pleased with their service and price. Inexpensive accounts wouldn't generally provide acceptable speed/bandwidth if you're planning to upload say multi-gigabyte video files or 400 mb Revit monsters, but if you're uploading "big" but not "massive" pdf sets, reasonable Revit, CAD, or video files, you should be fine with a low-dollar host.
Another very reasonable alternative is DropBox. It's not an FTP site persay, but it accomplishes the same job. It might not look quite as "professional" as ftp.yournamehere.com, but it's free, easy, and there's a good chance your clients already have it installed.
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Jonathan Holley, AIA, LEED AP BD+C
Gould Evans Associates
Lawrence, KS
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