Hi Caffey,
It does seem a bit backward to travel into Paris and back out to Villemomble, but this is pretty common as Paris is the hub of all train tracks.
In my estimation, judging from an
itinerary available via Google Maps, the price of a taxi would be around 40€. (A taxi into Paris itself is around 55-60€ and there is only 3 minutes difference by car between CDG to
Gare du Nord and CDG to Villemomble).
There may be some shuttle bus options for Villemomble, although, I'm not too well versed in that area. Most shuttles offer service into Paris at around 20€ per person. If a shuttle existed for Villemomble, I'm guessing it'd be around 15€.
Best of luck,
Ben