I don't know of any way to come up with a meaningful number for this given the limited information you've provided here. If these providers are practicing all over the country, then you are looking at very different markets with different demand, patient populations and payer mixes, so I'd expect to see a wide divergence in contract rates and reimbursement across this group. I'd note that even your Medicare and Medicaid rates are going to vary from state to state, since those rates are adjusted based on practice expenses and malpractice costs in the particular localities, so even if you used a percentage of the Medicare as your rate, then you would have to ask 120% of which state's Medicare?
It's not really clear to me what is the goal or purpose of having such a 'rule of thumb' number for such a large group - are they looking for guidance on coming up with a fee schedule or a way to broadly estimate payments for the group as a whole? Since it sounds like perhaps you are being asked to come up with a number that is an average of reimbursement rates for the group, not considering these variations by payer or location, my suggestion would be to just look at your existing reimbursement data for these providers. If you can have someone query the billing database for paid claims in the last 6 months for the top 10 or 20 most frequently billed CPT codes, for example, you may be able to calculate an average allowable per CPT code and do a comparison of those fees paid against a particular Medicare fee schedule and use that information as your baseline for estimating payment.
Hope this might help some, or if you can give some more detail about why this information is needed or how it is going to be used, perhaps we might be able to give a little more guidance.