I do things a little different.
I have a fixed price for the website. This typically is a couple of hundred pounds, then like @johntabita; I use a monthly retainer. For me this is around 15GBP and it goes up depending on how much flexiblity they wish. For instance, if a client want's to have a specific functionality which is included in a premium plug-in, then there monthly fee would be adjusted to be able to afford the yearly support plan with the plug-in. Therefore, if the plug-in needs 50 dollars per year for their support, then I increase the 15GBP to 18GBP per month, so I can cover the plug-in support and my time for supporting this. So this will be 50% above the normal plug-in support plan would be included in the monthly retainer.
In terms of your travelling costs. I've never had to recently travel to clients. Similarly to @TechnoBear; I work only with emails and phone calls. If the client really demand I travel to them then they'd need to be billed for this. I can do my job without needing to travel to them. If they really demand this, then they should be prepared to cover both my travelling expenses and time.
PS: I once traveled to a client 5 times for something which could have been done via email and video conferencing. Unfortunately since then I would never travel to a client unless they are willing to both pay for me travelling expenses and time.