So you have a front sight? And want to buy a rear sight?
$60 will easily buy the excellent Magpul MBUS2 rear sight. For a RDS, you can catch the Bushnell TRS-25 on sale for around $80, I couldn't recommend any RDS cheaper than that. I use this combination on a couple ARs with excellent results.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek