If you are paying 20% you need to shop around immediately! I think I am paying 1.75% or something like that for Visa, MC swiped transactions. If it's a non-swiped transaction it goes up to 2.38% or something like that. If it's a corporate card it's closer to 3% I think.
In answer to your question though, there is no way I would pay anyone 20% of my ticket for their processing services.
Now in answer to the op's question. Yes, this is worth doing but shop around on this. As for machines, it's been several years since I looked into it but I elected not to use any of the "portable" processing machines available at the time because it was my understanding that they were all pretty much as reliable as a cell phone. I never wanted to get caught not being able to run a card because of no signal reception as often happens with a cell phone. I actually leased to own a regular card reader that you would find at a retail store location. It's a little less conveniant than a wireless device because you will have to plug it into an active phone jack as well as a power outlet, but the system has worked well for me. In the event that I am unable for whatever reason to swipe the card on sight, then I just do it the old fashioned way. I write down their info and do it when I get back to the office. There is a slight risk in doing it that way, if the card is no good, but I have never had a problem.