yes, i agree - ask.
other resources that can help: 1. you can google average salaries for your level and field (this can be useful justification if you feel like the salary offer is really low), 2. you can also google specific people's salaries at public universities (which can tell you whether it is worth negotiating a lot or not (e.g., if your offer is the same as what all the other junior faculty make, they are less likely to raise it much), 3. use a cost of living calculator (available online)… particularly if you are going to a more expensive place than where you currently live, or if you have multiple offers in places that differ in cost of living, 4. if you have multiple offers, you can be up front so that the new school knows what number they need to beat (although maybe don't show your cards until you get their starting offer??).
also - often people underestimate how important starting salary is. as another poster said, it determines retirement & raises for the rest of your time at that school, but it also determines the starting salaries you will be likely to get if you ever change schools. you want to start as high as possible - even if you will be living in a cheap place and are okay right now on a smaller salary, your future prospects in that location or others will likely be tied to this starting salary. so for example, if you decide eventually to change schools, often a good argument for salary if you are going to a more expensive place is something like "cost of living is 16% more there, so i would like a 16% increase in salary (which is essentially "matching" your current salary). so the amount of this starting salary (that the 16% is based on) really matters. even if you leave academia, you can always negotiate higher salaries if you can say "i need you to at least match (or beat) X, which is my current salary".
obviously, there is only so much negotiating you can do and you want to be tactful and polite, but if you find yourself deciding whether to try to push for more start-up OR more salary (often they will only budge on some things)…. i recommend focusing on salary once your basic start-up needs are met.