After looking around and seeing what you can buy, and how cheap it was built, or how out of reach expensive they where, I built my own.
It's a 300 gallon tank that had been used for food syrups that cost me $100. Make sure it was used for something in the food industry and not a solvent industry. Smell the inside of the tank before buying it, there is no way to hide the smell of oils or whatever was used in it.
The trailer was also $100 but needed tires and a new hitch. Tires where about $60 each and the hitch was something like $50.
My first pump was just a cheap 12 volt pump that worked great until I bought a hand held sprayer for doing my fence lines. Then I learned that you needed a special type of pump for that type of operation. I'm guessing that I paid $100 for that. The hoses are fuel line hoses with two sprayer nozels at the end of a piece of scrap pipe that gives me about 12 feet of good spray and a little over spray after that, that I cover again in my next pass. That all cost around $20 or so. I ran a some 14 gauge romex from the pump to a cigarette lighter that I put on my tractor, and that came from the factory on my Mule. I pull it with either, depending on what's available. The tractor works better, but I sometimes don't want to take off the mower just to spray.
To get it to flow out the back better, I put some blocks of wood under the front end, and hold it in place with a ratchet strap. I estimate that with the tilt of the tank, that I put 200 gallons in it when I spray. I buy a 2.5 gallon concentrate of a Roundup type weed killer from Atwood's for $50 and dump it all in while filling it with water. Two weeks later, everything is dying nicely, and in a month, it's all dirt where I spray.
So for under $500, I have something that has already lasted me over ten years, and should easily last another ten years or more.
I use it mostly for fence lines and my roads, but also use it twice a year for my food plot to kill everything before disking and planting.
I figured out my speed to spray by trial and error. I want to go fast enough to not waste it, but not so fast that there isn't enough getting on the plants to do anything. This is another area where the tractor is better then the Mule. Low range, third gear works perfectly for me on my tractor.
Hope this helps.