As Tyler has suggested you can get significantly lower prices on $ per volume basis from Ag suppliers than from retail outlets.
One caveat on this is that you’re likely going to need to buy a larger volume than might be used in a single application in a domestic-type setting. You’ll need appropriate equipment to handle the concentrate(s) and then need to store the rest of whatever you may have purchased. Although these chemicals are relatively safe to handle you do need to observe precautions as unintended spillages can cause a range of undesirable outcomes.
With respect to storage I’d strongly suggest reading both the MSDS and the product literature to properly understand what is required. Many Ag chemicals need to be stored in darkness in temperature stable conditions. If you don’t do this they degrade much faster and have significantly reduced efficacy or, in some cases, unintended outcomes. It’s not much good buying something at 1/4 the price if it’s gone off by the time you use it.
Another source of wisdom on maintaining a low cost patch of green can be greenkeepers working at golf courses. The things they use have to be relatively inexpensive, very effective and not cause unpleasant reactions to people using their facilities or the native bush. A few years ago I was put in touch with a firm that supplies to golf courses through a golfing friend and I’ve been pleasantly surprised with the products.
Regardless as to what you use I would suggest finding out what the active ingredients are and comparing the prices on a dollar per gram basis. It can be worth asking people in your local area what they use and how it works for them. Sometimes things get swanky labels and marketing guff but there’ll be a generic product available at one tenth the price. Other things are absolutely brilliant at a particular job, cost 20 times the price of what you can buy at a retail outlet but are absolutely worth it.
For example, I use a particular set of commercial weedicides to kill blackberries. They’re more expensive, but I’ve found the generic products to be only 50% effective whereas these are 95% effective. I’m happy to pay a little more if I can get the blackberry under control a couple of seasons earlier and other plants established rather than wait what can be a further two years to establish cover.
