If cost is an issue...you are right, the OR is one of the cheapest ways to go....but indeed, what about sights? If you want irons on there, you'll pay for those - if you want high qual ones like Troy or similar, you'll pay $200 for the set.
I had a similar quandry myself - in the end, I just decided to go for the M&p15T - because it was configured how I would probably have ultimately ended up doing a cheaper version myself anyway, and for less money.