Hi K.,
While I can understand the guilt factor of spending all the money and time away from home, I just thought I'd throw in my two cents that if you're like most other moms I know, you probably deserve a little splurge, and $100 every couple of months, along with a couple of hours on your own is probably well deserved.
However, I also know the feeling of wanting to save, both time and money. So here's my take on at home hair-dying.
I've almost always done it at home, and no one has ever been able to tell the difference. In fact, often times I get asked where I get my hair colored because people like it.
I use Feria, but most of the brands are pretty good.
A tip that I read somewhere is to put Vaseline around the edges of your scalp so that the dye doesn't adhere to the skin (it's easy to look like you painted your hairline for the day after you dye it).
Other Tips-
1.Use a towel over your shoulders that you're okay with getting dirty and stained.
2. have plastic clips to hold your hair up while it's setting in the color.
3. Cover any light colored cabinets/areas that you wouldn't want stained... an old towel is good. (just in case you drip)
Have fun!
(i'm not sure about going lighter, as I tend to go with a slightly darker shade myself)