most economical way to get from CDG to Latin quarter?

Have you asked your hotel?

Since you didn't say the name of your hotel, or where it is (Paris is a big city), it is impossible to give a precise answer.
The information from your hotel, or their website, will almost certainly quote a métro station name as well as a street address. It is normal for establishments in big cities in Europe to quote the nearest métro/U-Bahn/Underground/.... station name.

Once you know the nearest métro station, you can look it up on the metro map here: http://www.ratp.fr/informer/pdf/orienter/f_plan.php?fm=pdf&loc=reseaux&nompdf=metro
Or the more sophisticated map showing streets here: http://www.ratp.fr/informer/pdf/orienter/f_plan.php?nompdf=metro_geo&loc=secteur&fm=pdf
Then you get the RER line 'B' from Charles de Gaulle airport, change to whatever métro line and emerge a few hundred metres from your destination.

You're asking about cheap, so there are two cheap ways.

First take the 350 bus from CDG to Gare de l'Est. From there you take the metro to your hotel. This is where we need Norma to give you the exact bus number from the airport, but I think she says it's the 350. Norma, come back.

Check out the price on the RATP website, the regional transport. Be sure to read all about how to buy the cheapest tickets.

The second cheap way is to take the new cheap bus from the airport, run by Easy Jet. It goes to somewhere inside Paris, maybe Porte Maillol. From there you take the metro to your hotel.

I think that new cheap bus runs to the Rivoli stop by the Louvre. The standard way in for budget travelers is the RER for 10 Euro but these bus workarounds are cheaper if more complicated.

...and the transfer from RER to the Metro is free.

Yes, Bets, it is the No 350 that terminates at Gare de l'Est. As I have endlessly mentioned, the fare is 3 tickets available from the driver, which comes to about 5 euro. The bus stops at all the terminals. Ask at the airport information desk (ADP) about outside of which door to wait for the bus. Usually every 20 minutes. The end.

And there is bus 351 which takes you to Nation and from there you can catch Metro Line 6 to get to the Left Bank. Depending on where you are staying in the Latin Quarter, this might be an option because you can connect at Place d'Italie with Line 7, which serves the Eastern part of the 5th arrondissement. One more thing, the "cheap bus" mentioned above goes to Palais Royal.

So for cheap, you get a book of ten metro tickets at the Tourist Information office upon arrival. That's 14.10 euro. Three tickets go to the 350 bus and one ticket for either the metro, line 4 to St. Michel station at the top of the Latin Quarter, or one ticket for the 38 bus from Gare de l'Est to the Latin Quarter, anywhere along Bd. St. Michel. Total: 5.60.

Or you take the RER and metro to your place on one 10 euro ticket from CDG to anywhere in Paris. You could take the RER direct to either St. Michel or Luxembourg, two RER stops that bookend the Latin Quarter.