If your desired salary was based on assumptions about their benefits that you've since learned are way off, then you do have room to say, "I know I said X is what I'd like to earn, but I've since learned that based on the difference in benefits, I'll actually be losing Y out of pocket." But here's the deal – it all depends on the range you're talking about. If your desired salary was $45k, they offer it, and the difference in benefits will cost you $5k, that's substantial, and worth mentioning and asking about – in fact, I would do it before they make a real offer if – and only if – you have a conversation with them before the offer comes. Most larger companies will have already sought the approvals for the offer amount and had it approved by HR by the time it is put in an official letter – then it's a little tougher to change, as you're asking the hiring manager to go back to the well. That is why it would be ideal to have the conversation during a verbal offer. I personally always make verbals before putting it in writing, since it's just not worth going through the red tape to get an offer letter out if you're not agreed on terms.

If your desired salary was $120k and the difference in real benefits cost/value to you is $3k, I'd take the job and smile…it all depends on the scale of the salary, and the scale of the difference in benefits.

If it's a small company, it won't be a problem. Over a couple hundred employees, it starts to creep into the red tape zone.

The one key I would consider is what your TRUE out of pocket difference is. That is, if your prior company offered matching to 6% and you didn't take advantage of it, I wouldn't argue the point too much. If you did, it's a big deal.

Also, my 5-10 percent suggestion was based on you not having given them a desired salary. Once you do, that option isn't really there anymore. That's why it's important to go in knowing what it's worth to you. If you feel like you undersold yourself on salary, I'd tread carefully about renegotiating on compensation unless there's a real difference in benefits value that you recognize.

Good luck, I look forward to hearing how it goes. Role play the conversation before it happens, so you're ready. At least with a mirror, better with a real person, even better with someone who hires as part of their role and is used to sitting in that chair.

As for your question about the value of benefits, I typically assume that the total package represents about 20-30% in benefits, 70-80% in salary. That varies, as benefits are not always highly variable. I would not think you'd need to ask your HR rep about it, but if you have the type of relationship where this wouldn't be risky, no harm in asking I guess. The real differences are going to be the ones you can calculate without thinking – huge differences in insurance premiums, 401(k) match, any special circumstance benefits (for instance some companies contribute to adoption fees or fertilization therapies, others don't touch either), etc.

Also, is it reasonable to ask my current HR director what is the overall value of my current benefits package (she knows that I am looking for a new position, and we are on good terms)? Perhaps she can help me understand the overall value of my current benefits to help me negotiate this with possible future employers. What do you think?
Thanks again.