No. 50g in Classic may be equivalent to $20 dollars USD depending on server, but $20 dollars won’t buy 175K gold on a retail server unless it’s through Blizzard.
If you could sell 50g for $20 dollars, and then bought a token with that $20 dollars, that would be a correct way to calculate the cost.
But, as has already been established, it’s not acceptable to trade gold for cash and vice versa. That means a potential token buyer has to compare the relative dollar cost of the in-game currency: ie, 175K retail gold costs $30 USD. Hence, someone looking to do this trade needing to convert classic gold into retail gold needs to use the gold > USD > gold pricing, which is 175K retail gold is $30 dollars and $30 dollars is about 120-150 Classic gold.
Caveats:
If you were going to buy a token for $20 dollars, then you could simply sell the 50g on the open market. You’d be in violation of the rules, though.
That’s why it needs to be gold to gold, with a common conversion factor ($30 dollars).
If you were going to do what I suggested above, however, you’d still be in violation when you did the transfer of 120g to $30 and then bought 175K gold with that same $30.
That means you have to find someone willing to do the exchange purely within a gold to gold framework, which increases the cost of doing business.
If you’re able to find someone willing to do the trade without going outside the rules, there is going to be a cost associated with that convenience. I don’t know what it is for someone else and I’m not even sure what I’d charge if I wanted to do it and that’s the great unknown assuming you are comfortable even doing such an unprotected and risky trade.