Can I patent self made Arduino shields??
You can patent a unique process or circuit or idea.
You can copyright your shield design and PCB design.
You can trademark your logo and company name/art.
But you just can't patent a shield you self made (not unless it has a truly unique idea, circuit or process).
I think the real costs of patents is not applying for a patent (expensive depending on the country you live) but to chase others that violate your patent.
100% correct. Same with copyrights. If you own a patent, and don't DEFEND your patent against someone who copied it, it's the same as giving up possession/interest in your patent. If a second company copies your product, and then you decide to sue that 2nd company, the 2nd company's defense can be you didn't sue the 1st company that made the same product/idea -- effectively giving up your patent claims.
... and this is the reason why big, giant companies sometimes has to sue the little guy. It's not the big companies are being mean... but they're protecting their patents. If another big company copies/use their patent, they can't claim you gave up your patent when you didn't sue that small mom-and-pop shop.
If you're a small shop, patenting is a waste of money. Just run like hell, quick and fast and produce/manufacture as much as you can of your product, be the first to market, and be the "authority" or known supplier for such-and-such product.