Bryan is a little more affordable than College Station, but I agree with Aggie98 - anything under about $700-800/month can be kind of sketchy.
Rent is really high in CS, so anything under $800/mo is usually a dump or in a bad neighborhood. Be weary of duplexes, they usually indicate run down neighborhoods or high volumes of college students. The duplexes are usually very close together and don't offer much in the ways of privacy or yards.
Just north of Georgia and Holleman, closer to campus (south of campus), are some nice and safer homes in the historic area around Brison Park.
Stay away from anything that says it's in Northgate! (north of campus and Dixie Chicken area) There are frequent crimes of people being held up at knife-point and gun-point there! Usually in the wee hours of the morning when students are leaving the bars, but still, not a place I would ever live.
Directly east of campus is kind of a mixed area with families and a lot of college students, but reasonably safe.
North of campus in the areas around Hensel Park and Tanglewood Park are nice, safe neighborhoods.