Your explicit question
Your question is about the statistics of startup offers to Associate versus Assistant Professors. I don't know if there's a resource that's able to answer that in detail, in general, across universities. The only Associate Professor startup package that I know of was indeed lower than most Assistant Professor packages, but I'm not in computer science, this Associate was bringing their own grant money, and N=1!
Your implicit question
However, it sounds like you're trying to use this data to make inferences about whether or not the university is giving you a "weak offer", and to use that information to set your negotiating strategy.
This is a bad idea, for a couple of reasons.
A "weak offer" is not interpretable.
Let's say the university is making you a weak offer. Normally they'd offer $X, but for you, they're offering $2/3X. Uh oh! What does this mean?
- They're not really all that excited about you. Sure, they'll hire you, but you were the best of a bad bunch. Not worth throwing money at.
- These are hard financial times. The new budget is much tighter than previous years, and there are hard limits on what they can afford.
- They don't understand your needs. Of course their previous hire, the robotics specialist, needed a lot of startup. But you, you work on computability theory, all you need is a pencil and a notebook, no problem!
- It is a lowball offer, but it's flexible if you negotiate hard. The other hires were given similar offers, but negotiated hard and raised it fine.
- Any combination of the above.
So, even if you did have an answer to your initial question, then you are left with even more questions.
Your startup needs are independent
Your startup needs are not dictated by what other people have been offered. Whether or not you need item X or software Y to succeed in this position cannot be determined by comparing your number to other people's numbers. This is a common problem in startup considerations, where the thinking is backwards.
Start with what you will need (and be generous, it's always good to have a little wiggle room). Come up with a budget to calculate what you would need to succeed, obtain grants, and really flourish. Share that itemized budget with your university contact. If the number is way off the number they are offering you, reflect on what is needed and what is flexible, and reflect on what is your best alternative to a negotiated agreement. That is, are you able to say "no" to them and walk away? Do you have other options? Can you maybe get away with using some cheaper equipment? Can you be creative with the startup, for example having it spread over several years (that way the capital outlay for the university is lower)? There are lots of options. Remember, the university has made you an offer because they want you. It is in their interests for you to succeed at your job and to flourish, so it is in their interests to ensure that you receive an adequate startup package.