i owrked at a plant that made a room about 12 x12 and 9 feet high, they lined three walls with 1 inch black pipe spaced about 10 inches apart. i think each wall was onits own zone to keep a good flow. tiwas a low pressure boiler delivering the water (i think it was water and not steam but i am nto real sure) the boiler only ran at 15 pounds. any way the room was always around 160 f and it was used to melt wax from those big garbage buckets you see on trailers. seemed to work ok bu ti dont know anything about the cost.
if you did this i would say ou would need copper or black pipe on the walls as they would give up heat much faster than pex? (again a guess on my part)