First of all congratulations, the Sig is a nice AR, one of my personal favorites. Vortex makes a good red dot. I might be wrong but I think it is in the price range. I have been really impressed with Vortex as of lately. Other than that I am sure these other guys might know of some others.
" The strongest reason for the people to retain the right to
keep and bear arms is, as a last resort, to protect
themselves against tyranny in Government...." - Thomas Jefferson