The Montparnasse-Bienvenüe metro station is practically two stations with a long corridor between them: lines 4 and 12 to the north, and lines 6 and 13 to the south. The train station is on the south side, so whenever you're boarding a train it's best to arrive by line 6 or 13. (Here line 4 is still faster if you're an average walker with no luggage, and you won't incur any extra stairs, just extra corridors.)
From Gare du Nord, the best rail connection is RER line B southbound (take any train heading in the right direction). Change at Denfert-Rochereau for metro line 6 towards Étoile (the interchange corridor is near the middle of the platform on line B IIRC). You will have to go up and down a few staircases. Plan on 30–50min from TGV platform to TGV platform, not counting any queue to buy tickets.
If you want to avoid staircases altogether (apart from a few escalators to reach the trains at Montparnasse), you can take the bus. There's no direct line between Gare du Nord and Montparnasse, and there are many combinations with one change. Off-peak, plan on 50–70min from platform to platform, not counting any queue to buy tickets. At peak hours, the bus can get stuck in traffic for a very long time (less so between the second Sunday of July and the next-to-last Sunday of August). There is also not much room for luggage on the bus, so I don't recommend it at peak hours. An advantage of the bus is that you can go to Montparnasse 2 TGV instead of Gare Montparnasse; Gare Montparnasse get you close to the rear of the train, while Montparnasse 2 TGV gets you close to the front of the train. A good combination to have the option to go to Montparnasse 2 is line 38 (departs from Gare du Nord so you don't have to worry about direction), then change at Port-Royal for line 91 (towards Montparnasse).
A taxi will be more expensive than public transport, but the 30€ figure feels large. I'd expect about 15€ off-peak on a weekday (more at night or on week-ends; 30€ might be reached if you get stuck in traffic).