I would not take my deal either because on paper even a Bonfire/Tamiyo is worth almost double what the expensive fetches are right now. You would be getting majorly ripped off if you think about the now and not the long term.
I understand that but give it 8 to 10 years and that is literally what those cards will be worth, if not more. The expensive Onslaught fetch lands are 40-50 dollars right now and the cheaper ones are around 20-30 dollars. All are very difficult to come by and to my knowledge they have only gone up in value since they where printed.
I have been hoarding new fetch lands just for that reason. I would not trade them for things like Bonfire/Tamiyo which is still being produced in mass quantity and in 2-3 years will more than likely either be a fringe playable worth 10-20 bucks (since it is mythic and only printed in one set as of now) or not be played at all and be worth squat.
If you can find a standard only player who wants to trade (they do exist) then good on ya. I'd love to trade anyone my bonfire (if I had one) for 2 Scalding Tarns. It is a fair trade right now. But us modern/legacy player's (who own most of the Zendikar fetch lands since the standard players want to get rid of them post rotation) would prefer to keep the older cards that are guaranteed to appreciate in value, are harder to come by, and are already being played in modern/legacy/vintage decks (I could be wrong about vintage, don't know the format to well). Bonfire is no where near as sure of a thing as the lands are.
That being said I have been wrong before and if post rotation Bonfires/Tamiyo's go in whatever the new awesome modern/legacy deck that comes around then 2 Tarns for a bonfire would be a great trade... but I would prefer to stick to the sure thing (and I have a feeling that most players like me would as well).
Good luck to you though - I hope you find some one who is willing to trade you the cards you want (they do exist, I have traded with them!). I meant this to be in no way malicious or anything, I just wanted to explain why I value the cards at what I do and also may explain why you have not gotten any bites yet on your post.
