According to at least one WoW player, the answer is that it is OK to trade gold in Classic to another person who will then trade gold in Retail to you. I do not claim any additional information than what is posted there.
It is noteworthy that another WoW player posted a link to what appears to be a conversation with a Blizzard Customer Service rep.
The OP of this thread though, seems to have asked what people think a reasonable price in Gold would be.
I think, based on an Internet search and a brief scan of prices that some sites are claiming they are selling WoW Classic gold for (something I would definitely steer clear from, as I am quite sure that is against Blizzard’s EULA), that 50g seems to be approx. USD $20, which I believe is comparable to the price of a WoW Token.
Whether someone would agree to give 175,000 gold in Retail for 50g in Classic (or vice versa), I do not know. Also, please note that I am not recommending nor endorsing doing this, and feel that it is important to note that if someone chooses to trade items in one game for items in another, they may have no recourse if that other person cheats them somehow.