Hey Paul, if you are bringing in a used bicycle you won't need a temporary permit or a customs broker. If you enter with your passport on a 90 day tourist visa you shouldn't have any trouble if you follow Philip's advice.
Some airlines used to sell special boxes for bicycle transport and some used to require them, but I'm not sure about their availability or regulations today. You will probably be charged excess and/or oversize baggage fees and they could vary greatly from one airline to another.
If you have temporary or permanent residency in Argentina you might get charged an import fee of 50% of the "value" of the bicycle. The best way to keep that figure as low as possible is to have a receipt for the bike with a price on which you hope to pay the customs fee if it is charged. In spite of a receipt the aduana can and will make their own valuation of the bike. If they know the cost in Argentina is $20,000 pesos they could charge you an import fee of $10,000 pesos, regardless of the price on the receipt.
In general used goods (especially clothes) are not subject to import fees. I think they would charge anyone (including tourists) who tried to bring in a "used" flat screen TV, but a used bicycle should be exempt if you have a 90 day visa. Don't show the receipt unless it's absolutely necessary. Insist the bike is only for your personal use while in Argentina and you will take it with you when you leave. The further "back in time" the receipt is dated the better. You can say you only brought the receipt with you to prove the bike is yours if there's ever a question about ownership...or a need to make an insurance claim when it is stolen.
I would think twice about bringing a bike that cost $20,000 pesos in Argentina to Argentina. Your bike would be an instant target for goons who would not hesitate to knock you down and ride off with your bike, not caring how badly you might be injured. I've seen bikes here that cost $2,000 pesos with all of the insignia painted over to make them less desirable to thieves. That would certainly make your bike worth less money as well.