Hello Paul,
I (personally) have consulted with 400+ U.S. Gardens, Museums, Zoos, Aquariums and other cultural institutions in this area and have helped many with exactly what you describe below.
There is no 'standard' per se, it varies greatly will all institutions and whatever the terms and conditions, it has to be fair and equitable for your Garden and the Garden's operator.
Please email me via artmanask@gmail.com and I will discuss with you at mutually convenient date/time, no cost or obligation. Happy to do this type of initial pro-bono service for AAM members. After we talk, if you still feel you need consulting work, I can refer you to consultant option(s) in your geographic area or provide you the support to be sure you receive what is fair for your Garden and the operator of your food concession.
Look forward to hearing from you.
Best regards,
Art
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Art Manask
Founder/Principal
Art Manask Consulting
artmanask@gmail.com818 358-3588
www.artmanaskconsulting.com------------------------------