I'm new at this myself, so I'm hardly experienced. However, I have already ordered liquid from 3 different vendors and I can already say, there's not only a difference in price but in quality.
I bought some RY4 from Cignot (Dekang, I believe - label is small and hard to read) along with my Ego-T systems - it's a nice dark yellow color and full of flavor. Cignot's a great company, great service, but prices are a little higher. Their Marlboro is good and strong as well, in my opinion. I ended up liking the little bit of sweetness that RY4 has more than the straight Marlboro. The Marlboro, to me, has a typical harsh cigarette taste.
So like you, I saw Mad Vape's much cheaper prices, so I ordered some RY4 from them. They were out of DeKang, so I ordered their own custom mix. Color is about 10 shades lighter and flavor about 10 times weaker than the RY4 I got from Cignot. Both were 24mg nic strength. So I won't be ordering from Mad Vape's again, unless they get DeKang back in stock. Maybe I got a bad batch, but I didn't care for their liquid at all. Fortunately, I have the other flavors from Cignot, so I am mixing them into the Mad Vape's liquid to use it up and get some flavor out of it. Mad Vape's Root Beer is excellent, however. Think this just goes to show that even from the same company, some flavors can be a hit, while others are a total miss.
So on Saturday AM, I ordered some DeKang from mikeusa.com. It hasn't arrived yet (it's only Monday as I write this) but it's supposed to be genuine DeKang and his prices, including shipping, are cheaper than Cignot. Cignot does give free shipping, I believe, if you order over a certain price point, but I wasn't, so I couldn't take advantage of that - that might have made them equal to or cheaper than Mikeusa if I had ordered enough to qualify for free shipping, but I didn't do the math all the way out. Anyway, like Cignot, I heard from Mike right away that he received my order and 20 minutes later I got another e-mail saying that he had already mailed it out by USPS priority, so I have to say, his service is great, just like Cignot. Will post back when the liquid arrives as to it's quality, but have read posts here that his stuff is good.
Anyway, don't beat yourself up with buyer's remorse. It's all a learning experience. We are all going to make purchases that don't work out so well at some point. Just keep reading reviews here and then, unfortunately, you will just have to buy different things from different vendors until you find a combo of taste/price/service that you like.
Good luck!