I applaud your direction and interest. But, I suggest you need a bit more focus on what your thinking of building. There are a LOT of grinder sizes and shapes and designs. Most grinders in the knife making world are defined by the belts they use - 2x72, 4x36, 1x30, 1x42...etc. All of those can be used for knife making. But, the grinder designs and features vary considerably. You need to study the variations in design and list what it is your requirements are.
Then, are you asking about building a grinder from uncut steel, or are you thinking of building a grinder from pre-cut pieces and then welded or bolted together? Do you want variable speed? If so, how do you achieve the speed change?
Before you ask any more questions regarding price, you need to specify what it is you are wanting to build. It's kind of like saying "I want to build a boat" without knowing anything more. Depending on the buyer price may, or may not, be a driving factor in purchasing a grinder.
Don't be discouraged by these comments, you will find a great number of people on this forum willing to help you. But, you've got to take this inquiry seriously and think about what it is you are asking.