Maybe by 10. Spirit and opportunity cost $400M each. I guess it all depends on how selective your sample is, and whether or not the return rocket takes the needed fuel with it. I'm thinking of just grabbing whatever scoop you can get, and blasting off. Spirit weighs 400lbs. I wonder what is the lightest rocket that you can get to escape Mars with 1lb of soil.
The trick is getting back up the gravity well. I'm sure Kerbal Space Program could tell you. The fact that the Soviets started their discussion with an N1 does not fill me with confidence that it can be done economically. Something something 16 tons of hydrogen delivered to the surface of Mars. Something something fully-loaded Falcon 9 on the fucking surface.
About 20 years back I read an article in Analog proposing that terraforming Mars could most easily be effected by causing an artificial greenhouse effect via carpetbombing the place with millions upon millions of megatons of thermonuclear weapons. It'd sure free up some sequestered oxygen and nitrogen, you betcha.