Premium items vary so drastically in value. Here is what I've seen recently:
Amnesia (3 gold): 10,000 to 15,000
Rainbow Jar (10 gold): 100,000 ~30,000 (thanks Pidove!)
Mutagen (3 gold): 15,000 to 20,000
Any magic fruit (1 gold) : 5,000 to 10,000.
Squad energy (15 gold) : ~100,000
One gold seems to be worth 10,000 silver for items with decent demand. However, I've seen squad packs selling for 300k, and invites go for 1000 or less (except for geneticists). Also, the other high-cost items like mercenary licenses and expand scrolls are almost exclusively offered for rare item swaps. I don't know what you would get more silver for, 50 gold or a mercenary...given how squad packs sell you could probably get 500k for either, easy.
But I digress. A straight gold trade is probably the safest bet so you don't have to fiddle with item demand. If I were you, I'd start with 10k per gold. You might scale it up since gold is much more convenient than any item itself, and any sort of a bulk deal should definitely be scaled up since gold in large quantities is difficult to come by. For 50 gold I'd start at 750k or 1mil to gauge demand, giving myself room for haggling without getting too close to its minimum value.
Sorry if I got off track there, I really like inspecting BK's economy! XD
Edit: I'm sure you've noticed this, but be careful about using the Item Exchange to gauge item value. It's full of bad trades!
Final edit: Altered price estimates to reflect discussion
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