I am putting together #6 rifle this week. An M4 kit from Del-Ton with shipping was right at $500. Paid $86 for a stripped Stag lower, and for less that $600 another black rifle (that will be a deseert camo painted) will be born.
You can also peacemeal the rifle together. The upper would probably be the highest cost single piece, but you can do it over time. I have done this when funds were tight, and overall, it costs more, but sometimes you have to be practical. Include ammo and magazines in your rifle build estimates too.