Well, if I may contribute a bit too, here's my 2 cents.
Now, I've setup a VPS running Cacti which for me is doing much the same, for me at least. The cost of running this seperate machine isn't much so to be honest I would not be willing to pay a great amount of dough for this particular feature. Also, cacti enables me to view the stats on my iPhone too. Although the graphs are very very slick on the dashboard, I can't view them on a similar slick way on my mobile device too.
Next to cacti I'm using the New Relic application too for monitoring my servers. So what Carl is stating is not that bad too.
Being a one man 'company' I'm spending money on a server here, backup there, a few dollars over there. In the end you're spending huge amounts of $ on many small services.
The actual choice of spending extra on a new feature again will be less easy if it will cost just too much which can be done yourself too with a little extra effort/cost.
Now, to be very honest, since there are many VPS competitors on the market today, I think it might be wise to offer such services as a standard to give you an extra edge. I mean the Linode dashboard is already a winner, but this might even be a extra sellingpoint. I mean, like me, many of your (to be) clients must be suckers for these graphs
