Lunar samples (rocks, soil etc.) were returned by US Apollo program, Russian Luna and Chinese Chang'e. How does the cost per kg of the samples compare among these missions? Who got it the cheapest per kg?
Partial answer, just for Apollo
Note: Using metric specified in OP's comment of total program cost / total mass of rocks
In 2019 US dollars \$175,000,000,000 / 382 kg = \$458,115,113 / kg
IMHO that's a ludicrous metric though, implying nothing else was gained.