I've only been asked that question in an interview twice, and in both I asked why that would matter. I walked both times (away from a decent offer in one case).
What's being tested is not 'how did your previous employer value you?', it's 'how good at negotiating were you when you started your last job?'
Additionally, the value of less numerical benefits gets substantially ignored in such calculations - I worked for two years at a job that paid 40k, but the flexibility, atmosphere, and environment were worth the 30k drop to me. There's no normal way to explain that in an interview without it sounding like an excuse.
Next time somebody asks that question, I'm going to tell them '15k and a few thousand bananas'.