I want a good but cheap flash for my nikon D3000 (and D800)?

I've ordered SB 900 but I want to buy a second flash that's quite inexpensive but good. I also want it to be compatible with Nikon's TTL system and usable as a slave. I was planning to buy Nissin Di622 Mark || but a friend of mine (photographer) told me it's not strong enough. What do you think?
Any other suggestions?

My budget is up to 180 dollars.
(I'm also planning to buy Nikon D800 in a few months)

If you get the nissin close enough to the subject you won't have problems… Depends what your shooting…

Look not a Nikon SB-400.It's about $120 and your SB-900 will fire it as a remote using the Nikon CLS system.

In that price range, the Sigma 630 ST is going to be the best option.

Why does your friend think the Nissin is not strong enough? It's much more powerful than the SB-400 or SB-700.

Nissin Di622 MKII is good third party flash not as powerful as the SB900 of course but for a second flash it will do.