Most advice pointed to taking RER "B" to Luxembourg or Port Royal, then heading east by bus or taxi. Then advice from a clerk at hotel (Devillas) says just take Les Cars Air France to Gare de Lyon, then switch to bus 91 to Saint Marcel La Pitie.
(two people @ 18 euros each). Counting the bus, that's about 40 euros total. Maybe the taxi would be better? This will happen 5 - 6 p.m. on a Wednesday. We are not experienced travelers, and we're not rich. Or stupid. Any thoughts on our options??
Since there's two of you, and you'll be carrying luggage, and since you seem uncomfortable with mass transit after a long transatlantic flight (completely understandable and rational), I'd recommend opting for the cab. Once you've been there and get more comfortable with the buses, Metro, and RER, you can consider using these for your return journey. I've seen very experienced travelers who wouldn't even consider any other method besides a taxi, so it's hardly "stupid"- just personal preference. See this thread for proof that not "everyone" takes the RER from the airport: http://www.ricksteves.com/graffiti/helpline/index.cfm/rurl/topic/87690/paris-and-france-for-two-weeks-three-questions.html
The one consideration is that at rush hour you will be sitting in jams watching the taxi meter go up and up. I'd opt for the Air France bus option, and then decide on taking a taxi or bus from Gare de Lyon. You'll get caught in jams on the bus but you won't have to pay through the nose because of it.
Your hotel's website, http://www.hoteldevillas.com/en/ actually gives the best, cheapest, and fastest way to get there: RER B to Gare du Nord, then Metro Line 5 (map color 'orange') in the direction of Place d'Italia to Metro stop 'Saint Marcel.' Looks like about a 5 minute walk from there.