burning all the way through thick cardboard is harder than you think, unless you set the laser down and wait a while. The ash turns whitish and then starts scattering the light. I was just doing this last night. Also be careful of highly reflective cardboard like video game boxes, they can be harder to burn than white.
If you want a good laser, I recommend you save an extra 20-40 bucks to break that difference, or get a used model.