Not all quotes are the same. This can be for a lot of reasons. For one thing, every company has a different amount of overhead costs that are required to keep the company running. These costs can include advertising, tools, professionally services, licenses, and so on. For a contractor to be successful they need to have this cost covered on any job that they do. There are a few items that are required by law to be a part of every contractor's overhead. These include, workers' comp (even if they have no employees), liability insurance, and taxes just to name a few. If a contractor's bid is exceptionally low or if they require cash as the only method of payment, these could be indicators that the contractor is not paying for these things and therefore operating illegally.
Another thing that causes variation is allowances. These are entered into a quote to give the customer a feel for how much then entire job costs. It's impossible for a contractor to know what faucet you will pick out and it could cost anywhere from $25 to $500 or more. Therefore most contractors will either put in an allowance or leave the item out completely and leave it up to the client to purchase the faucet. Be sure to get a concise list describing the scope of the quote and a breakdown of any allowances that are built into the price.
And then of course there is quality. This includes not only the look of the finished product but also what goes on within the walls and under the floors. The granite countertops and travertine tile do not hold up your house nor do they protect it from water, heat and cold, or fire. Building codes are a complex thing and are constantly changing. If a contractor is unwilling to get the necessary permits for a job then consider that a red flag. While they do add expense to the job, they also provide a level of protection for both the homeowner and the contractor. Small handyman jobs may not require a permit but anything that affects structure, changes the plumbing or electrical system, or adds or removes finished living space require permits. And in the state of Ohio, even if the contractor said you didn't need one, the homeowner will be held legally responsible for renovations being done without a permit. When in doubt check with your local building department.