Can someone tell me about how many would be needed in a stealth cabinet or closet size grow box?
To me, a stealth cab and a closet are two way different sizes. I would say:

Using good parts and efficient drivers, 50 watts of LEDs will match 100 watts of CFL, fluoro, or HID.

Costs about $2/watt for the LEDs plus $1/watt for the drivers.

So, for $180 you can build a 60 watt LED light that'll match 120 watts of something else. (A 90-watt UFO will NOT replace a 400w.HPS. That's a sales pitch. 200 watts of high-end emitters and drivers, though....no problem.)

Not really cost-effective (yet) unless:

a) heat is a big problem
b) electricity's real expensive, or
c) maximum stealth (no electrical or thermal footprint) is critical.

I'm probably gonna put together a DIY tutorial if anybody's interested in building their own light. Let me know, Hermie.