At least where I live, solid core slab doors are $40 at Home Depot. You could almost certainly build a desk top from lumber that would be cheaper, but not by a significant margin.
I don't see them on Home Depot's website any more, but they weren't pre-drilled, pre-hung, pre-primed, or mortised for hinges, none of which matter for desk tops, but perhaps that's why they were that cheap. I thought about replacing some of the hollow core doors in my house with them, but I don't trust my ability to install anything more than pre-hung doors. I already had a motorized standing desk, so I wasn't in the market for a cheap desk top, either.
In contrast, the pieces I mentioned above are $16 ($6 ea. for the MDF, $2 ea. for the 2x4s)