It's more a risk assessment problem than a cost effectiveness problem. They're not going to get funded for a 50-100kg fully-instrumented UAV (and the boosters a

It's more a risk assessment problem than a cost effectiveness problem. They're not going to get funded for a 50-100kg fully-instrumented UAV (and the boosters and spacecraft to get it to Mars) until they prove the concept first. The brass is willing to spend a half million on a mission that piggybacks on and minimally impacts a currently-funded mission to find out if UAVs are feasible. The reality is Opportunity has covered ~45 linear kilometers in its 14 year lifespan. They hope to cover hundreds of square kilometers with the UAV approach.