That is a serious exploit in the economy. What diamond vouchers are worth is a LOT more than what $20 would get you in Rupees. So you can buy the vouchers then sell them for a LARGE amount.
Unless the gold trade is in one of the first 7 trades that infamous 9th trade of the gold has a 1 in 10000 chance of showing up breaking the infinite paper villager every time you use it.
To my knowledge the option to buy rupees is instantaneous, so you basically get what you pay for: instant cash. A voucher otoh. can make you more money but only in the longer run. And you'd still be depending on a buyer. Don't forget that some players are online at times where 4 - 8 players on one server is common. Which makes finding a potential buyer for your vouchers in-game a lot harder. Sure; they could auction the vouchers off and thus make more money, but that also wouldn't be as instant as the option to buy rupees is. So I wouldn't call this an exploit, there's a difference to be kept in mind here.
I also don't see it as an exploit. This is an economy server and economy is based off supply and demand. The rupee value of a diamond voucher is based off that and could change at any time. If there was an abundance of diamond vouchers around the supply would not be as great and the prices would fall dramatically... possibly even below what you purchase $20 worth of rupees off the site. It just recently happened with beacons. Beacons were starting to be sold in excess of 20k. Someone on one of the SMPs posted he had 75 beacons for 16k each. Guess what that did to the beacon prices. So, like I said, I don't think it's an exploit... It's business.
I think you're better to sell your emeralds to a mall as it's instant cash. I buy loads at 19500 for example. Or trade the emeralds to get even more valuable items like glowstone or armour.
$20 is the fixed standing price, but don't forget that a) rupees were cheaper a few months ago, and b) incentive vouchers were given out a few months ago. I would like to see rupee prices come down. It's almost as if the price is so high as to not compete with player shops
