If you want inexpensive, you can go with a Magpul MBUS rear, but NOT a front... they are not for gas block mount (they'll melt), but that could save a bit of cash if need be and you aren't concerned with matching.
I have some Midwest Industries rear folding sights that work fine, I have no problem recommending them. Decent price for a decent product. Just be sure to get the right height front sight... you'll need the taller one intended for gas block mounting, not the shorter ones intended for handguard height rails.