I haven't seen any for under $100, and you'll also have to hope the measurements of the mannequin are close to your own.
I actually use a quarterstaff. I run it through the arms and hang the hauberk between two things; used to be the top of my bed when I had the canopy bed, now I use a pair of portable workbenches (or you could use sawhorses).
If there's somebody to help, the easiest way is just holding it close to tight, then lacing it closed, then cutting the links underneath the lace. If there's nobody to help, you can do the "count the rings" method - I'm using that one on my current hauberk tailoring. I made a string of rings, only one wide, and then you use it almost like a cloth tape measure, to measure different parts of your arm. It will look best if the tapering is linear, and not in and out a lot. I tested my tailoring by zip-tying the rings together after cutting out the excess, then trying on the hauberk to make sure it still fit. I have to finish mirroring the changes on the back side of the other sleeve, then rivet it together, but it's otherwise done.
IMO, a mannequin might help to tailor it to the size of the mannequin, but not to yourself. I can take some pictures of mine as I go along if it will help any.