Overall, I think you're right. Of course, Gary is more qualified to comment on this issue than either of us.

I'm pretty sure that the MER spacecraft were about as big you can make and still launch to Mars on a Delta II booster. If you got much larger, you'd have to use a more expensive booster. That probably wouldn't have fit in an already tight budget. Sometimes, though, going with a more expensive and powerful booster might actually save you some money because you're aren't trying to squeeze the most out of every gram. That works in theory, anyway, but an engineer's natural tendency is to maximize the payload regardless.

The Viking spacecraft were much bigger than MER. Each Viking had both an orbiter and a lander. Both were inserted into orbit together with a big thruster burn that required a lot of propellant. Each of the Vikings were launched on a Titan III booster. I don't know for sure what each launch cost back in 1976, but they were many times as expensive as a Delta II. They don't make Titan IIIs any more, but the closest comparison is the Titan IV which costs upwards of $400 million per launch.

The airbag landing technique does impart some heavy loads on the payload. I've read that during the initial impact, the MER payload pulled 40 Gs. The G load dropped with each bounce, but it bounced many times. That's tough on a payload, especially if you're talking about sensitive instruments. Given the distance that MER traveled after impact, it doesn't exactly sound like a precision landing technique.