Just noticed this thread Andy. I used to have a fairly large Reseller hosting business, which I sold just last year. At one point we had over 250 accounts and charged from €200 to €2000 per annum, as it included a lot of support and management. We did everything from e-mail setup, SMTP service, database admin, security, content management, domain registration, site upload. Mostly because it was attached to a website company turning out up to 80 sites a year. It made sense at the time, kept us in touch with the clients and added to the bottom line. But unless you've a very good platform, then it's a nightmare!
At the time we were using a reseller account that gave us a H*Sphere master control panel originally setup with Hostin365. We had two - Linux and Windows. We could define packages (e.g. Starter, professional) and then setup pricing for additional services (databases etc). So we could create individual accounts for each of our clients with separate logins and FTP etc. Only cost about €400 per month (I set it up in 2003) and it was split across multiple shared servers and MySQL servers. Worked really well (until the company was sold, H365 I mean).
It was a good model and it worked. I nearly had a second go at it with a company I co-founded in 2010 but one of the directors unfortunately gutted and stripped it behind our back. Ah well. Drawing board etc.