Cost is very subjective. I buy Nikon grips because it IS worth it to me. IMO, better construction and more reliable. That being said, many users have had great success with third party grips. I'm not sure which ones excel so you should probably read reviews to get an idea.
I also have Nikon grip for my D7100. And I have no complaints.
But like other post there are some 3rd party grips for half the cost.
But I don't have any idea how they hold up.
The Nikon grip I have has held up really well for me.
I, too, prefer the real McCoy of Nikon grips. I have them on two of my cameras...the D200 (MB-D200) and the D300 (MB-D10). My D40 is the only one with a third party grip, made by Aputure (BP-D3000). No complaints so far with it.
I have a Nikon grip for D7000 and it was great. Just got a D600 and wanted to save so got a third party grip, it was loose and the green battery light kept flashing every time it flexed. Changed it for the Nikon one and glad I did, fits like a glove and works fine.