When making scripts, I set a minimum of $10 per script plus $15 per hour (if it's really simple, $10 per hour) but that's
per script so if someone requests a system that needs 3 simple scripts and 1 complex I'll ask from $55 to $70. There's some people that pays for conversion, then I'll ask to see the scripts before, and if I can use any of them I'll ask $10 per hour (without the minimum of $10), if not, I'll ask the same as if making new scripts. If there's future errors because of my code, I'll fix it for free.
When making source changes, first I ask what source does the person uses and if he want me to work directly with his sources (most people will not want to send you the sources). No matter how simple the source change, I always set a minimum of $25 plus $20 per hour. Why: If you make the source change, and for some reason that does not involve your code, his server gets a segmentation fault, he'll look for you. When this happens, and the error is not in my code, I explain to the client (without complicated terms, unless asked to do so) and won't charge anything on the first time. If the seg fault/error is because of my code, I'll fix it for free.
For any kind of support, it's really hard to give a price. Most people will pay for support when they're dealing with a segmentation fault, and on older sources with amateur changes it can be quite a headache, so I wouldn't recommend doing support services unless the client gives you a sure fire way of reproducing the error.
PS: For source changes when the client doesn't send the source, when sending the code most of the times I ask for the clients specific files, upload it to a secret gist (gist.github.com) and then upload my changes on his files. You may want to use TeamViewer, as it can happen of the client messing with the syntax without knowing when doing the copy paste and thus unable to compile.
Sorry for the wall of text, hope it helps you