Not to be difficult, but 'reasonable charge' is pretty subjective. If you have parts you really want, the easiest way to convince someone to help you might be to provide the most detailed explanation and description you can of the part. Most of us make parts purely for enjoyment- if it's not fun, we probably won't do it.
-If it's a simple quick/easy to do part, you might easily find someone willing to help you.
-If it's more complicated you might find someone who will decide that your part is interesting, and might be a fun challenge.
-If you can't provide everything that needs to be known about the parts, or design/guesswork is required to make it, you're going to have more trouble finding someone to make it. I personally would charge like crazy for this, if I was willing at all to try.
While compensation will very possibly be appreciated by whoever makes the parts, you may be unlikely to entice a hobby machinist to make stuff for you by compensation alone. Show us what you want/need- maybe it will interest someone.