My daughter does Tae Kwon-Do (she used to do karate but after a kid almost got hurt, the instructor decided kids were too much of a liability so we had to find a new school). I pay $350 a month and they pick her up from school, take her to the dojo, do homework for an hour, and then they do the class, so it's an aftercare/martial arts class, hence the high cost. No contracts (this is something you may want to check up on, as kids sometimes lose interest and then you're stuck in a one-year contract, only you know how serious your kids are about passions/hobbies). She was offered a free introductory class (after she had the chance to sit through a few classes to see if she liked it, the instructor, and the kids in it), and she liked it enough that she has been there since about March/April.
I used to join her at the same dojo and do the adult Tae Kwon-Do class later in the evening, which was 3 times a week, at $65 a month, but because I recently became separated, the expense was just too much for me to bear in addition with all the other living expenses that I must shoulder on my own, so I ended up quitting, unfortunately.
Based on the effort and patience that goes with caring for multiple kids, getting them to concentrate and not play, and the fact you had an extra day on top of my 3-day schedule at the price I paid for myself as an adult, I'd say you're getting a great deal, but I would have your kids sit through some classes, as well as asking if they can get a free class or 2. Some dojos offer up to 3 free classes, but most will offer at least 1, especially since everyone knows how kids can change their mind so quickly.
I don't know if your school charges you per belt, but I have been to 3 different martial arts dojos and all of them charged whenever I advanced a belt - it was about $40-$50 extra. You also need to buy uniforms. Some schools throw in a uniform when you enroll, while others charge extra for the uniforms, so that is something else to consider and be prepared for. Tournament registration is another expense, and driving. Most of the tournaments tend to be on the West Coast of Florida (Port Saint Lucie), or even Orlando.
I think martial arts is great, it teaches kids discipline, self-esteem, respect, and of course, self-defense, and it is a good way to stay in shape. My daughter's instructor won't allow the kids to advance belts if their school conduct is low, they misbehave in school, get bad grades in school, or act up at the dojo, so it's not just about how much they know in terms of martial arts, but how much maturity, discipline, and respect they show in their daily lives. Same if the child acts up with his parents and the instructor witnesses this or is told by the parents that the child misbehaved at home. It really is a path to personal growth.
I wish my mom had put me in martial arts as a kid, as I had to fight off perverts and aggressive boys on dates sometimes as an older teen and in my 20s, and I wasn't very good at self-defense. I think it's a great tool, especially for smaller kids who may be picked on or taken advantage of. It will help keep bullying in check, as you're taught to use martial arts as a last resort, while walking away is your first resort.