Once you run the bill of materials to build a commercial quality product you'll find that doing it it strictly for cost savings doesn't make a lot of sense. Pins, cylinders, hoses, couplers, steel, welding wire, gas, plasma tips, etc all add up. OEMs get volume discount on materials that the general public can't replicate. Don't get me wrong, I'm all for building stuff but it usually works out cheap(er) because I use scrap materials, used or surplus cylinders, etc. Plus I can tailor the design exactly to my needs be it lighter, wider, stronger, etc than commercially available tools. With an "open source" design or a few similar designs you make compromises to satisfy differing requirements which drive the cost back up.
I wish you good luck with your venture.
If you are wanting to design and build a grapple because you cannot afford a new one.....you may get the designing done but you still arent gonna build one because you wont be able to afford it.
Same way with alot of other implements, and a pretty common one is a log splitter. You just cannot build one for less money than you can buy one for UNLESS you already have some of the material.
I can buy a good functioning grapple that will do everything you can ask a tractor with 1800# lift to do.....for $1100 all day long.
Building a one-off grapple is gonna cost you that much, in material.....then you are gonna be working on it for a good week cutting and welding and fitting parts.
I am all for building your own stuff. It gives you something to be proud of. But just want you to realize it isnt going to be a cost savings. So if that is your sole motivator....quit while you are ahead.
Now if you like projects, and enjoy being in the garage planning, laying out, cutting, welding, painting, etc. And dont mind doing all that work for no savings at all.....and strictly just to be able to say you built "that".....then go for it.