Hello Sojourner...
I spent 3-4 months doing a lot of research before we moved from California the first part of April. You are very wise to question those things now. Many do not seem to take into account the cost of anything except housing before they move. Here are a few answers that I hope help.
http://www.dot.state.tx.us/vtr/vtrreginfo.htm?pg=faq
This is the Texas Dept. of Transportation website FAQ. Texas goes by age of vehicle when they figure plate fees. Plus, there's a $90 new resident tax and additional taxes from each county (I think Dallas county even charges a bridge tax). Getting the first year plates for a newer model car would be approximately $200+ and that won't include the emissions inspection... usually $39.50. Driver's license is $24.00 for 6 years.
$390 for a nice 2 bedroom in a lovely landscaped neighborhood is highly doubtful in Dallas or any of the suburbs. Triple that and you would be closer although there are some nice senior communities (but I have no idea of their cost.)
One thing that I had a very difficult time getting figures on were utilities and food. What I did find was that utilities seemed to be sky high, obviously due to it being *HOT* here during the summer and the strong need for air conditioning during those times. I have yet to get my first electric bill but I do expect it to be a little higher than what I paid in California due to KWh differences.
Food seems to be a little higher than it was in California but I'm sure that will vary for all parts of the country. What you could do if you are still considering moving is check out some of the weekly ads from the local grocery stores... Tom Thumb, Kroger and Albertsons. If they require a zip code for their ad you can always give them your son's zip code so you can see the local flyers.
Sorry,.. I can't give you a figure on car insurance because we lived in a large metropolitan area so ours went down.
Personally, after reading your information and knowing what is here in the Dallas area, I would suggest that you stay put. I think your dollar would go further where you are than it would here.
Best Regards,
Lany