You'll be hard pressed to find anything that is "good" for under $100 in the way of grinders. As for what is workable, many people start with a Crafstmen 1x42". However, it's not ideal, and it will take some time to get use to.
If money is a factor (and it is to all of us), then you can get a great start by hand filing. You can build a filing jig to guide you and file in your primary bevels. From there, you draw file (sit with the knife pointed towards you, and draw the file from the plunge to tip while holding it perpindicular to the blade). This will get you a great flat, but it takes lots of time and elbow grease.
A 1x42" is definitely a start, and it will remove steel faster than hand filing. From there, it's probably a good idea to go ahead and draw file to get things truly flat.
--nathan