I have a 2.5' x 2' x 6' space, and I've had several different lights in there. It started with CFLs, then went to a low level Mars burple LED light, for a while it had a 4000k LED Shop Light in it, and now it has a pair of Optic 120's in it. The Mars was the least expensive at $75 delivered. The shop light was actually $200 from Home Depot (its in my shop now, veg's nicely but not very potent for flowering). The Optic 120's were about $325 each, so I have $650 worth of COB-based pre-made LEDs in there now.
You could build a DIY COB light and probably get away with just two Cree 3070's, and it can be done for under (or around?) $200, I think, haven't priced them recently. I have a friend with a 2x3 grow cabinet, not sure the height but I'd guess around 5' tall. He did one grow with just two 3070's, and then added two more for the next grow. He did very well both runs.
How many watts is the HPS you are replacing? How many grams per grow have you been getting?