Personally I wouldn’t go over $55 for one of the T-Promo sets. All of the ones I’ve purchased have been under $55 and they were all Mint/sealed. I purchased the Imakuni set for roughly $35, the Tyranitar set for about $15, the Eevee evolutions for $25, etc. I’m referring to sets not including the comic.
Those are really good prices. Like Y!J prices 
Those are also the prices that I’ve gotten for all of the sets that I’ve bought; I refuse to pay more than $20 for the 2nd stage starters, as individual cards from those sets appear on ebay for ~$5 a pop every month or so - the only downside is that you may have to live without the white "Trainer’s Magazine Vol. ___) card that came with the rest of the set.
Less popular ones like the Slopoke set, and the starter evos are worth about $35 a set. Of course the Legendary Birds ($80+), Dragonite line ($60+), and Imakuni($90-$100+) sets are worth the most and can go quite high.
Most were from eBay, only ones from Y!J were the Eeveelutions and the Imakuni.