the $50 rails are usually the drop in kind (not free float) and they seem to be heavier than the $250 ones.
I have a UTG and a Troy. The UTG was $40 and the troy was $80. Can't really tell the difference. For mounting lights/lasers/grips those are fine for me. A Daniel Defense Omega is what I really want on my next one.
I'm told that a free float is better for long range accurate shooting.
Keep an eye out over at ar15.com or cheaperthandirt.