If a you are looking into a complete DIY LED build, the cost can be quite low. The biggest physical challenge with larger lights will be the chassis itself. I built a nice little ~40 watt reef LED for my nano tank, it would have cost me less than $20 CAD.
There is a lot of reseach involved to get the right combination of LEDs to suit your needs. There is also getting the right drivers, getting or building a PWM circuit to allow for dimming, sourcing the hardware, math, building the chassis, etc.
How big is the tank you want to light?