You seem to be jumping immediately to the most expensive options for staying fit. A super expensive gym, your own treadmill, P90X which is just about one of the most expensive workout DVDs you could choose. Most people never really use those options even after they shell out their money on them. A lot of people make that mistake, which is why we have a very lucrative fitness industry while still having a lot of overweight people. This might sound harsh, but it's the MMM forum, so I assume you can handle it: you've gotten sucked into thinking you need to buy things to stay fit, when what you really need is a good dose of prioritization and the willingness to look past your own excuses.
Here's some other options for you. Some may swing toward the female demographic because they're things I've personally used, but I'm sure dudes can chime in if that's not your thing:
Zuzka Light (youtube) and Bodyrock (.tv) both have free/low-cost videos workouts that, if done right, should kick your ass. You can improvise for most of their equipment- heavy sandbags, etc. You will also see a lot of fake-boob cleavage, which might be a good or bad thing depending on your perspective I guess.
You can also find a lot of free bodyweight workout plans just by googling, like this one:
http://www.benderfitness.com/2012/10/full-body-fitness-workout.html Stumptuous.com has lots of information on weight lifting, and several sections on how to make your own at-home weight lifting equipment (sandbags, DIY power cage, etc.). You can also get barbells and weights fairly inexpensively on Craigslist.
Your local library might even have fitness DVDs and books for FREE. I've gotten a lot of stuff at mine, which is awesome because I usually get bored with them right about the time I have to return them anyway.
Another vote for running, walking, or biking outside. Lots of people have small children and manage to do this, it just takes some forethought on your part. Take the kid for a walk with you in the evening. One of you can get up early while the kiddo is still sleeping and workout. The other one can get a run in after he/she goes to bed. If you prioritize fitness, I'm sure you can find enough time to make it happen.