We went with Flybe who go to Charles de Gaulle from Cardiff twice a day. Eurostar is better if you live in London, and if you are able to get cheap deals and have plenty of time it's worth doing from Cardiff. From the airport you need to get a free shuttle to one of the other terminals, both of which have train stations, then it's a €17 return trip to the Gare du Nord on the RER Line B train. The hotel was Villa Fenelon - I've stayed there before, it's good value, no curfew, and conveniently located about halfway between the centre and Montmartre (it is actually within walking distance of the Gare du Nord). The nearbyHotel Monte Carlo has better decorated rooms but doesn't have the nice courtyard, prices are similar.
Drinks in the clubs are expensive (though the €12 entry to the WAGG club includes a drink, most of the drinks there being €6). You often have to pay a compulsory cloakroom fee as well.