The Viking program cost several billion dollars when adjusted for inflation. That's roughly 10 times as expensive as each of the current missions. You might as well try to compare a Suzuki to a Rolls-Royce.

The Viking missions used radioactive isotope generators to produce electical power. Those generators add a lot to mission life but they also add to costs. The Viking missions were launched on Titan 3 boosters that cost more than the full mission cost of the current missions. Even the billion dollar budget of the early 1990s Mars Observer mission was enough to guarantee success. It died a few days before it was to enter orbit around Mars. It wasn't enough to ensure that the Hubble mirrors were properly ground or that the high gain antenna on Galileo would deploy properly. Both of those errors seriously hampered the scientific data returned by the spacecraft. The Hubble repair mission that fixed the problem cost over $500 million.

The current generation of missions include a lot of innovation largely due to budget constraints. One innovation is the use of aerobraking to achieve the proper orbit. If they used propellant to do that (like Viking did), the costs would be much higher because all of that extra propellant would mean you'd need a much larger and more expensive booster than the Delta IIs. Remember, it takes propellant to accelerate propellant and weight tends to snowball rapidly. As a rule, aerospace vehicles tend to cost by the pound, with increased weight leading to higher costs.

Another innovation is the use of the airbag technique for landing. This is a pretty robust technique that reduces the required propellant needed for landing and also opens up a lot more potential landing sites. The Viking landing sites were chosen largely for safety rather than scientific interest.

The whole point of these lower cost missions is to do a lot more of them than we can afford if we go with the gold-plated billion dollar systems. For the cost of a single one of those giant vehicles like Galileo or Viking, you can fly a bunch of the new generation missions. Personally, I'd prefer to see several missions to Mars, asteroids, comets, etc. over a single mission to one place. However, we have to find the balance between faster, better, and cheaper. In reality, you can pick any two.