SS,
I think there are a couple things intertwined in your question here.
If you're at a decent salary, most insurance companies will assume (or know if they know your company) that your company provides some sort of long term disability insurance. A lot of times insurers will only try to estimate the gap between company insurance and salary.
The premium costs are not based on what you make but rather the likelihood of you using the plan. Based on how healthy you are, the job you work etc, companies take that risk into account and determine at what level they will be profitable. Different companies see you differently/focus on different industries/ have different risk profiles which creates the range of premiums.
As an aside, if your company policy covers your expenses (at no cost to you) it may not be worth it to purchase another policy, spending money to protect you from a non-existant shortfall if you wer disabled.