I wanted to trade in my 10.5 pro and BB was offering $250 for trade and i was able to get $351 @ Apple. I'm paying tax at either one and $101 is $101, so I was good with that value, considering the going rate on eBay.
If you can wait, there will almost surely be a sale on the new Minis within 30-60 days. If you're looking to trade an iPad in, I recommend checking in daily on Best Buy's site (Services -> Trade-in) to track the trade-in value of the device you are planning to trade in. That's what I do when I'm looking to trade in.
Generally, Best Buy's trade-in values aren't the best, but their trade-in values DO vary from time to time and can inexplicably increase temporarily. I've seen them rise as much as 40% only to drop days later. Sometimes they coordinate these trade-in value spikes with the announcement of new devices. But since the device isn't on sale, it's a wash over waiting for a sale.
I've got my eye on the Mini 5. The current trade-in value for a 32GB Mini 4 is $110 (as of this moment). I'll be tracking it... should it rise, I'll trade it in, and hold onto the gift card until the Mini 5 goes on sale. I expect to be able to get the 64GB Mini 5 for a net of $175.
Depends, so, for example, the 2018 iPad Pros went on sale in early November and right now Microcenter is discounting them up to 200.00 off depending on if its the 11" or the 12.9" Model and what size storage and if its wifi or LTE.