we offer two (well three) options for maintenance when we finish developing a clients site.
1) Content Management System - We develop a "custom" CMS for the clients site and put it in /admin. when he logs in, he has access to change the content of specified parts of the content. AS far as prices, we charge $20/month in addition to their hosting cost + a setup fee (customizing our CMS to fit their site). the setup is dependant on the extent of the site. This is for more of the "Do it yourself" kinda people, but don't want to worry about messing up the design.
2) Prepaid Maintenance - When they need a change to their site, they fill our the maintenance request form, we do it, and send an invoice stating what we did, time spent and time remaining. They prepay a set amount of hours per month (5, 10, 15, 20...) and pay for it at 1/2 our normal hourly rate (50/2 = 25 per hour). If they don't use all their hours, then it does not roll over. if they go over, they pay for the extra time using our normal hourly rate (50 per hour). So if they bought 5/hrs per month for a year it would be $1500 (25*5*12). This is best for those expecting updates often, like ecommerce sites, or like to know exactly how much they will spend each month. This is the plan we prefer.
3) Pay-Per-Update - basically the same as the prepaid one in setup (they submit maintenance request form, we do it and send invoice) except when we send the invoice they need to pay that amount. This is NOT at a reduced rate, so 5 hrs of work = $250.
hope that helps. we make about 50% of our money from maintenance which is great. for design, you usually need to go out and find new clients. with maintenance its recurrent income. However, i run a design studio and offer hosting only as an added extra (we aren't trying to make a profit on it, just break even).