Ok, now I have time to break down for you what you just said. This reply is also directed to the OP as well.
You are right, $10 is 25% of $40.
The thing is that the girl isnt getting $50. She is maybe getting $20... and more than likely in this example $10. In the OP's example with his little tantrum... she gets $5.
Now, whether or not you agree with how the business is being run... do you think this is fair? A woman is doing her best, using her muscles and massaging your body for 30 minutes. Maybe you didnt think the massage was great, and thats your valid opinion as we all have had bad massages... trust me I have. But in the end, you tipping her $5 is nothing short of a slap in the face.
I am with you about the structure of the business. I don't think it is run fairly, or ethically. But, unfortunately for me and for us and most importantly for the MPA's that's the way it is done. There are many other businesses that I do not agree with their models as well. We in Ontario get charged a significant "delivery" fee for our hydro. Yet we pay our bills or we have our power cut off. We pay carbon tax on gasoline and diesel, we pay land transfer tax when we move to a new house, and for those of us who are fortunate enough to be able to afford multiple properties, we get slammed with paying capital gains tax when we sell one, which is a hugely significant number. We pay for those things. Are they fair? Nope. Are they money grabs? YEP.
Now, OP, you say that you are not a rich guy, so your money is very important to you and how you spend it matters to you. I understand that. I spent a big chunk of my life in that situation so I can relate. Maybe this hobby isnt for you then. I want you to think about her side too though. I can almost guarantee that in your head you think that she makes a ton of money. I assure you she doesnt. She is probably in the same situation as you. If your $5 matters to you, it matters just as much to her. So that is why she had the reaction that she had to you not wanting to tip. There are two sides to every situation. You may not have the funds (again, im going by what you said only, not being judgmental in any way) because you are working yourself in an unfairly compensated career. If thats the case, maybe try to reflect on how someone in whatever work situation you are in attempts to cut back your earning potential. That would probably upset you too right?
My last point is this. You went to a place that is set up to provide massage with extras, expecting to get a good regular massage for cheap. Sometimes you get lucky and get someone with legit skills who is working those places because they can make more money by providing a little extra or have more foot traffic. Most of the time the massage portion is secondary however. That was your big mistake. I understand wanting a good massage, actually that is 100% my thing too, and I often get disappointed, you can read about it in my reviews here too.... but I would never take money away from someone who is working hard to earn a living. Regardless of the quality, I always would tip the minimum... and if it was better I would tip accordingly. If a good massage is all you want, it has been mentioned that you should go to a legit place then. MayBlossom has a bunch of locations, and they dont expect extras tip $. I'm sure there are lots of others people can mention too. Those places tend to have more fair payment systems for the girls, as the extras are not available and that tip money isnt being paid. This visit should have been somewhere like that.
If you continue with this type of attitude towards women in this industry, especially now knowing their circumstance... I think you will find it isnt the hobby for you. Burning bridges will only lead to you having poor visits, and thus wasting your hard earned money. Treat them nice, and they return the favour. That doesnt mean you have to be extravagant by any means, it just means understanding the system, adjust your budget accordingly, and enjoy yourself.