There are areas of Coppell that are old and also relatively new. I have lived there for over 20 years. You can find houses from 250K to 3 million. There are some areas not as affluent as others but the median house price is now over 400K and the average family income is fairly high. There just aren't many FSA deadbeats around here. It is mostly built out, very little land or lots available so not much new construction is going on. Brand new construction that is happening will run you around 450-500K and up but they are really nice.
If you have kids, all the schools are quite good from elementary all the way through high school for both academics and sports. Crime here is very low with the occasional bicycle getting stolen.
Go to realtor.com, use Coppell in your search criteria and see what is out there. I recently saw a couple for 275-300k that were pretty nice. Even considered one of them myself. Having said all this, it is very much a sellers market in all areas around north Dallas. Nice houses in Coppell can and do sell in 1-3 days and it is like that in a LOT of areas. I know of no one who has lost money on a house around here. During you research, if you have questions about specific pockets or areas, IM me any time.
ETA: we have a fairly nice golf course, a couple of good lakes in the area if you are a boater and Grab-a-Gun is just down the street from me.
Coppell is like many areas, you can have 400-700K homes on one street and two streets over, they are 200-250K. Just be selective.