Send what you can genuinely afford without creating a hardship for yourself first. That is the only rule that actually matters here. Inmates are fed three meals a day regardless of what is on their books. Nobody goes hungry because their account is empty. What commissary money buys is comfort and small pleasures above the baseline: snacks, hygiene items beyond the basics, phone time, postage for letters, and in some facilities access to email.
Most facilities cap monthly spending somewhere in the range of $300 to $340, and that ceiling does not include phone time or electronic messaging, which come out of separate account structures depending on the facility. So the practical upper limit for commissary alone is already defined by the system.
For starting out, a modest amount gets them what they need for the first week or two while they get oriented and figure out what they actually need to buy. Something in the $50 to $100 range is a meaningful head start without requiring you to sacrifice your own financial stability. You can build from there once you know what the regular needs look like.
The most important thing is consistency over time rather than a large initial deposit. A reliable monthly amount they can count on is more valuable than an unpredictable large sum.