I found this post on negotiating job offers pretty enlightening.
As far as checking market value, I like the Stack Overflow Salary Calculator and then sanity-check it with other sites like glass door.
One of the points that the first link makes is that not all compensation is about money, and sometimes you have to focus on other aspects that are important to you. For example, in a bootstrapped (as opposed to VC) startup there probably is not much money to be had and devs are huge overhead. (In that, it might take a while to launch the product and realize value from devs.) They may not be able to pay market value, but they might have a product you believe in, might let you work from home full time, pick your dev tools, bring your pet to work, etc. But in an established enterprise environment, you might get paid pretty close to market, but you probably won't get those other benefits and you must use X Y Z W and Q tools. Every time. Even when it doesn't make sense. You also will probably have to chase non-sense metrics to give executives the false sense of security they crave or another data point to justify what they were going to do anyway.
Point is, salary may be the most important point, but it is not the only aspect of compensation to think about. Compare it to what you believe is market value and weight the total compensation vs the job environment.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.