The unit is the part of a disposal job most people look up first, and it is often the smallest part of the work. What sets the scope is the sink, the power, and the old unit. Read those before you settle on a model or a budget line.
Start with power. Look under the sink for an outlet and check whether it sits on a switch you can reach. If there is no outlet, or the outlet is in the wrong spot, a trade may need to run a new one, and that changes the scope. Ask whether the quote includes any electrical work, and whether a permit is required where you live. Local rules decide that, not this guide.
Next is the drain. Check the sink flange, the trap, and the pipe below the sink. If a dishwasher drains through the disposal, the connection and the knockout plug matter, and the manual for each unit names the parts. A drain that needs new pipe, a new trap arm, or a different fitting adds both work and parts.
Then look at the mount. Most units fit a standard mount ring. A sink with an unusual opening, a worn flange, or a stainless sink that needs a new mounting seal can add steps. Ask what the quote assumes about the sink, and whether any change to it is listed as an extra.
Access matters too. A sink over a cabinet floor, a shared base, or limited room to work takes longer. Plan time for removing the old unit, which may be stuck or hard to reach, and for hauling it away. Ask who removes and hauls the old unit, and whether that step is included.
When you compare quotes, compare the scope line by line. Look at the unit, the mounting, the drain parts, the power work, the old unit's removal, and the haul-away. Two quotes that look different may cover different steps, and a lower total can leave out the outlet or the drain fitting. Ask each provider to write down what is included and what would be extra.
Ask about the paper as well. Find out whether the unit's warranty is handled by the seller, the maker, or the installer, and what it says about installation. A warranty that depends on a particular installer's steps is worth reading before you sign anything.
For the typical service life of a disposal and how to plan a replacement around its age, see the garbage disposal page.